A state is a set of properties for a layer. The properties can, for instance, define how a button should look when it’s ‘pressed,’ or a menu when it’s ‘open.’ You can switch from one state to another state with or without animation.

‘pressed’ and ‘open’ are example names. You can pick any names you want, but a layer will always also have a state called ‘default’: the set of properties it had when it was created.