Who and what the sentence is about called what?

The "who" and "what" of a sentence are called the subject and object, respectively.

* Subject: The person, place, thing, or idea that performs the action of the verb.

* Object: The person, place, thing, or idea that is affected by the action of the verb.

For example, in the sentence "The dog chased the ball," the subject is "dog" and the object is "ball."