Actors are objects in the game world (as well as many non-physical objects that aren't real objects but simply store data) and contain functions and data pertaining to that object.
For example, a pawn is a type of actor which represents a character in the game world. It contains all relevant information such as location, rotation, the current weapon. It has functions which are relevant to the object, such as those which allow control over the character. A gametype is a non-physical type of actor which is simply used by the game to manage the rules. It contains data such as game time, score, players, and so on. Another example of an actor is mutator.
The vast majority of classes in UE are derived from actor, although some are derived directly from object.
- Actor on beyondunreal.com