Combat is organized in a cycle of rounds and turns. A round represents about 6 seconds. During a round, each participant takes a turn.
Certain things let you take an additional action on your turn called a bonus action (only one per turn).
You can briefly communicate on your turn and interact with one object or feature of the environment during your move or action.
Certain things allow you to take a special action called a reaction: a response to a trigger of some kind, occurring on your turn or on someone else's.