Everything a creature does in combat turn is codified in terms of abilities. Each ability, in turn, has one of four possible maneuvers that determine when it can be used.
Active Maneuvers
Active maneuvers are done proactively within the time scale of a single turn. On each creature's turn, they may take up to three Actions in total. Every active ability will specify how many of these actions it consumes. They may be single actions, double actions, or triple actions. Some active abilities may also be free actions. These not only don't consume a creature's actions, but can be done at any time (even on other creatures' turns)! However, they cannot be used in reaction to other abilities.
Reactive Maneuvers
Reactive maneuvers can only be done in response to some event that triggers them. Every reactive maneuver will specify what its triggering conditions are. These could be taking damage, hearing another creature speak, another creature moving away from you, or anything else. Each creature has only one reaction per turn. However, some reactive abilities are free reactions. These have no limit on the number of times they can be used, so long as their triggering condition is met every time.
The order of reactive maneuver resolution is determined by a "last in, first out" stack. So if so if a creature casts Fire Ball, and another casts Dispel Magic, and the creature that first cast Fire Ball casts Dispel Magic on the first dispel, then the Fire Ball resolves. A given creature can only use a given reactive ability once within a given stack.
Passive Maneuvers
Passive maneuvers act in the background and don't require any decision to use. passive abilities will always have a duration listed that tell you when they are and aren't in effect.
Slow Maneuvers
Slow maneuvers are those that take longer than a single turn to execute. Some may be on the scale of a short or long rest, in which case they cannot be used in combat. Other slow abilities, such as most rituals, have a more precise execution time (or casting time) specified in seconds, minutes, hours, or some other defined unit of time. These can be done during combat so long as a round doesn't pass without the creature executing them using Sustain at least once.