During combat you may do following:

You can move at any time during your turn (before, after, or during actions).

You can interact with one object or feature of the environment for free.

You can take a bonus action only when a special ability, spell, or feature states that you can do something as a bonus action.

A reaction is an instant response to a trigger of some kind, which can occur on your turn or on someone else's.

Conditions alter your capabilities in a variety of ways, and can arise as a result of a spell, a class feature, a monster's attack, or other effect.

Effects that obscure vision can prove a significant hindrance to most adventuring tasks.

The presence or absence of light in an environment creates three categories of illumination.

Some creatures have extraordinary senses that allow them to perceive their environment.

Obstacles can provide cover during combat, making a target more difficult to harm.