A head noun is:
The word that is modified by an adjective clause.
Want to see an example? Look at this sentence:
In this sentence, car is the head noun because the modifies it.
Usually the head noun is the noun that comes right before the adjective clause, but not always.
Look at these examples. Can you figure out the head noun?
1. The people in Japan I met were very polite and friendly.
2. The CD in the purple jewel case that I was just listening to is not very good.
3. The TOEFL prep programs here in the USA that I visited seemed quite helpful.