call is a built-in meta-predicate that allows its single argument to be called/invoked as a goal. For example, a program might create a goal (perhaps using =..) on the fly, and then, presumably later in the program, need to test the goal. Here are queries that perform these roles - in a real program, both the assert and the call would be built in to Prolog procedures written by the programmer.
?- assert(likes(mary, pizza)).

?- call(likes(Person, pizza)).
Person = mary

?- Goal =.. [likes, mary, What], call(Goal).
Goal = likes(mary, pizza)
What = pizza