Attack
Make one melee or ranged attack. If a feature gives you Extra Attack, you attack twice (or more) whenever you take the Attack action.
Cast a Spell
Cast a spell with a casting time of 1 action. You can't cast two leveled spells in one turn — if you cast a bonus-action spell, the only other spell you can cast that turn is a cantrip with a casting time of 1 action.
Dash
Gain extra movement for the turn equal to your speed (after applying modifiers). Great for closing distance or fleeing.
Disengage
Your movement doesn't provoke opportunity attacks for the rest of the turn. The safe way out of melee.
Dodge
Until the start of your next turn, attacks against you have disadvantage if you can see the attacker, and you have advantage on Dexterity saving throws. You lose the benefit if you're incapacitated or your speed drops to 0.
Help
Lend aid: a creature you help gains advantage on its next ability check for a task you're helping with, or advantage on its next attack against a creature within 5 feet of you, before your next turn.
Hide
Make a Dexterity (Stealth) check. You need to be unseen — you can't hide from a creature that can see you clearly. If you succeed, you're hidden: unseen and unheard, with advantage on attacks against creatures unaware of you.
Ready
Prepare an action for a trigger you describe ('I shoot the first goblin that comes through the door'). When the trigger happens, you can use your reaction to take the action — or move up to your speed as part of it. You must choose the action now and hold it.
Search
Devote your attention to finding something: make a Wisdom (Perception) or Intelligence (Investigation) check.
Use an Object
Use a second object interaction, or use an object that requires an action (drinking a potion, for example).