What is basic concept of Overheads?

Suppose a student study 7 subjects in CA IPC. He purchase books for each subject and takes classes for each subject.
So Can you find out cost per subject? Answer is “NO”. Since what about cost of PENs which he uses for all the subject. “How the cost of PENs should be distributed over 7 subjects” is the subject matter of chapter Overheads. Overhead means charging of all indirect cost over main products using different basis for absorption.(Overhead absorption rates)