DnD 5e - Time
For your characters, time passes just like it does for you: seconds, minutes, etc. proceed in linear order. However, much like a video game or a movie time for your character typically doesn't perfectly line up with real-world time. Stretches of in-game time will often pass in a real-world instant, while crucial seconds in the game world may take considerably longer in the real world.
While neither of these terms are used in the official rules text, it's helpful to think of two methods of tracking time: Narrative Time and Combat Time.
Narrative time is often abstract, and is used when players are performing a task which takes a long time (more than a minute), but the specifics of the task typically don't require narrative attention. Traveling, sleeping, eating, and other mundane, routine tasks are great examples of when narrative time comes into play. Sometimes these tasks will take fixed amounts of time (8 hours for a long rest, 1 minute to descend an elevator, etc.), but in other cases the Dungeon Master may need to roughly estimate how long a task takes.
How many minutes a task takes typically doesn't matter unless the players are under some kind of time pressure (I'm in a house that's on fire, maybe I shouldn't spend 5 minutes reading this letter), so a rough estimate is usually fine. If you have a rough idea of the hours passing throughout a day you're usually fine. Tracking longer periods of time, such as days, can be more important. If events are happening in the world, tracking the passage of days helps to track when those evens occur. It can also be fun to look back on the weeks, months, and years of a character's career and see where they started and how far they've come over time.
For more on Narrative Time, see pages 32-33 of the Dungeon Master's Guide under "Tracking Time".
Combat time is strictly measured in 6-second "rounds". During a single round, every creature participating in the combat has a "turn" in which they act, each occuring in order based on the creatures' initiative results. Creatures continue to act in this order until combat ends. Regardless of the number of creatures acting, rounds always take 6 seconds and turns occur one at a time. While this may defy real-world logic, it's an effective abstraction of real-world time.
More on rounds, turns, and initiative will be covered in the Combat section of this guide.