Answer:
Who refers to the subject of a sentence. A subject is the person, place, thing, or idea the sentence is about. Whom is the object of the sentence.
Example:
Who: “That's the girl who scored the goal.” It is the subject of “scored” because the girl was doing the scoring.
Whom: As the objective pronoun, receives the action. For instance, “Whom do you like best?” It is the object of “like”.