DnD 5e - Playing the Game?
This section gives a basic overview of how playing the game works. More detail on specific mechanics like combat and magic will be explored in later sections.
The Gameplay Loop
The basic interaction of the game can be though of a "gameplay loop", occuring in three steps:
First, the DM establishes and describes the state of the world. Depending on the situation, this will vary in scope and detail. In some cases the DM might set out the state of the world at large, describing the world which the player characters inhabit, and where the characters are in the world. In some cases, it will be more specific, such as describing the state on contents of the room where the characters currently are. The DM will also describe anything which is currently happening, such as non-player characters doing something or a rock falling.
Second, the players describe their actions. Depending on the scenario this might be a strictly defined action like attacking in combat, or it might be more freeform, like having a discussion with a NPC or examining an object.
Third and finally, the DM describes the outcome of the player's actions. This may be an objective mechanical response such as a player hitting or missing an attack, or a player succeeding or failing on an ability check, or it might be a more narrative response like telling a player that the object they're examining is warm to the touch.
After the DM describes the outcome of the players' actions, the loop begins again. The DM can describe any changes to the state of the world (the DM need not repeatedly describe the size of a room, for example), and can add any new information.
In practice, this interaction is less formal and much more conversational. Actions and reactions are a persistent back-and-forth between the DM and the players which takes place verbally and which periodically includes rolling dice.
Types of d20 Rolls
d20 rolls are the most important and the most frequent rolls in the game, and they cover any determination of success and failure. These rolls come in three types, each of which has a different purpose.
Most of the time when you roll a d20, you will roll against a target number (either Difficulty Class (DC) or Armor Class (AC)). In all cases, ties go to the roller. So if you roll a d20 against a DC of 10, you need to roll 10 or higher. Remember: "Meets it, beats it."
Sometimes the Dungeon Master may call for a "contest". Contests take place when characters directly oppose each others' actions, such as when they are arm wrestling. Both characters roll and add their modifiers, and the higher roll wins. In the event of a tie, neither player succeeds and nothing changes from the current state of the world.
Ability checks cover most of the things which a character attempts to do which require a d20 roll. When you make an ability check, you will roll a d20, add your ability modifier, and compare the result against a DC set by the Dungeon Master.
Ability Checks and Proficiencies
Sometimes when you perform an ability check, one of your proficiencies may apply. In these cases, you add your Proficiency bonus to the check in addition to your ability modifier.
In some cases, the DM may call for an ability check with a specific skill. Any character may attempt an ability check, even if they are not proficient with the skill, but without proficiency you do not add your proficiency bonus to the check. These checks are denoted in writing by the ability score's name, followed by the specific proficiency in parentheses. For example: Wisdom (Perception), or Intelligence (Mason's Tools).
When you attempt to hit an enemy with a weapon or with some spells, you will make an attack roll. When making an attack roll, you will roll a d20, add your attack bonus, and compare the result against the target's AC.
Natural 1s and 20s
When making an attack roll (and only when making an attack roll), the number on the die may have additional effects. If the number on the d20 is a 1 (a "natural 1"), you automatically miss with the attack. If the number on the die is a 20 (a "natural 20"), you automatically hit, and you score a "critical hit". For more on critical hits, see Damage, Healing, and Dying, later in this guide.
When attempting to resist or avoid some effects, you will make a "saving throw". These effects usually come from special abilities like a dragon's breath or from a spell like fireball. When you make a saving throw, the DM will tell you what type of saving throw to roll, and you will roll a d20 and add the corresponding ability modifier, then compare the result to the DC of the effect.
Saving Throws and Proficiencies
Characters are proficient in some saving throws; typically you gain two saving throw proficiencies at 1st level which are determined by your class. When you make a saving throw of a type in which you are proficient, you add your proficiency bonus to the roll in addition to your ability modifier.
For example: Fighters are proficient in Strength saving throws. If a fighter makes a Strength saving throw, the character adds their Strength modifier and their proficiency bonus to the d20 roll.
Advantage and Disadvantage
Sometimes a circumstance will make it especially easy or difficult to attempt a task. If something makes a task unusually easy, like attacking an enemy who can't see, you may have Advantage on the roll. If something makes a task unusually difficult, such as attempting to fire an arrow during a windstorm, you may have Disadvantage on the roll. Some abilities explicitly grant Advantage or Disadvantage, but usually the decision is up to the Dungeon Master.
In any case, you can only have Advantage once and Disadvantage once, and if you have both they cancel each other out. So if you have Advantage from a dozen different sources, but you still have Disadvantage from just one source, you roll normally as though you had neither.
Under some circumstances, the Dungeon Master is encouraged to give a player Inspiration. Inspiration is an expendable resource, and you either have it or you don't. It can't be accumulated like coins or something.
Inspiration is a valuable resource. You can use it to grant yourself Advantage on an Attack Roll, Saving Throw, or Skill Check. You can also grant it to another player, which can save another player's life if they're facing an important saving throw.
When to grant Inspiration is vaguely-defined, and entirely up to the DM. As an example, the Player's Handbook recommends granting Inspiration whenever the player does something notable which fits their character's Personal Characteristics (their Traits, Ideals, Bonds, and Flaws), or when they do something which contributes meaningfully to the story.
As a personal recommendation to DMs: use Inspiration frequently. Most DMs use it only rarely, which encourages players to hoard it until they need to pass a save to stay alive. Use it as a reward for making the game more interesting and fun, and encourage your players to spend their Inspiration and continue to look for ways to earn new Inspiration by doing interesting things. Inspiration is the easiest rewards that the DM can grant, and it has little long-term impact on the game so it's easier to award than xp or items. Be generous with it.
For more information on Inspiration, see the Inspiration section in the Personality and Background chapter of the Player's Handbook, just before the Backgrounds. This is an abundantly horrid place to hide this rule, and many new players overlook it.