On a creature's turn, they may make up to three actions. Moving, attacking, and so on typically use some number of these actions. Creatures also have one reaction that can, in principle, be use at any time within a round. They regain their reaction at the start of their turn. Creatures cannot "save up" their actions or reactions between rounds or turns. Actions and reactions reset regardless of whether or not they're used.
In addition, some effects may begin, apply, or cease at the start or end of a creature's turn. These are specified within the relevant rules text.