An enum is an abstract datatype with values that each take on exactly one of the finite set of identifiers that you specify. Enums are typically used to define a set of possible values that don’t otherwise have a numerical order such as a suit of card or a particular season of the year.
Ex:
Public enum season(winter, summer, spring, fall);
Season s=season.winter; // define it into a new file to make it global
System.debug(s);