Initiative determines the order of turns during combat. The combat rules in “Playing the Game” explain how to roll Initiative.
Sometimes a GM might have combatants use their Initiative scores instead of rolling Initiative. Your Initiative score equals 10 plus your Dexterity modifier. If you have Advantage on Initiative rolls, increase your Initiative score by 5. If you have Disadvantage on those rolls, decrease that score by 5.
Surprise. If you’re Invisible when you roll Initiative, you have Advantage on the roll. If you’re Incapacitated when you roll Initiative, you have Disadvantage on the roll.
Action. On your turn, you can take one action. Choose which action to take from those below or from the special actions provided by your features. These actions are defined elsewhere in this glossary: Attack, Dash, Disengage, Dodge, Help, Hide, Influence, Magic, Ready, Search, Study, Utilize.
Reaction. A Reaction is a special action taken in response to a trigger defined in the Reaction’s description. You can take a Reaction on another creature’s turn, and if you take it on your turn, you can do so even if you also take an action, a Bonus Action, or both. Once you take a Reaction, you can’t take another one until the start of your next turn. The Opportunity Attack is a Reaction available to all creatures.
SRD 5.2.1, pp.184–186, also p.176 · CC BY 4.0