Dexterity

Dexterity measures agility, reflexes, and balance.
Dexterity Checks
Make a Dexterity check to move nimbly, quickly, or quietly.
Attack Rolls and Damage
You add your Dexterity modifier to your attack roll and your damage roll when attacking with a ranged weapon, such as a sling or a longbow. You can also add your Dexterity modifier to your attack roll and your damage roll when attacking with a melee weapon that has the finesse property, such as a dagger or a rapier.
Armor Class
Depending on the armor you wear, you might add some or all of your Dexterity modifier to your Armor Class.
Initiative
Initiative determines the order of turns during combat. When combat starts, every participant rolls Initiative; they make a Dexterity check that determines their place in the Initiative order. The GM rolls for monsters. For a group of identical creatures, the GM makes a single roll, so each member of the group has the same Initiative.
Hiding

The Game Master decides when circumstances are appropriate for hiding.

With the Hide action, you try to conceal yourself. To do so, you must succeed on a DC 15 check while you're Heavily Obscured or behind Three Quarters Cover or Total Cover, and you must be out of any enemy's line of sight; if you can see a creature, you can discern whether it can see you.
On a successful check, you have the Invisible condition while hidden. Make note of your check's total, which is the DC for a creature to find you with a (Perception) check.
The condition ends on you immediately after any of the following occurs: you make a sound louder than a whisper, an enemy finds you, you make an attack roll, or you cast a spell with a Verbal component.

Passive Perception is a score that reflects a creature's general awareness of its surroundings. The GM uses this score when determining whether a creature notices something without consciously making a Wisdom (Perception) check.
A creature's Passive Perception equals 10 plus the creature's Wisdom (Perception) check bonus. If the creature has Advantage on such checks, increase the score by 5. If the creature has Disadvantage on them, decrease the score by 5. For example, a level 1 character with a Wisdom of 15 and proficiency in Perception has a Passive Perception of 14 (10 + 2 + 2). If that character has Advantage on Wisdom (Perception) checks, the score becomes 19.