it means to be full or unlimited e.g. Politicians have plenary power

Because of its importance, the case was presented at a plenary session.

A Plenary painting is a painting that is totally complete. It is considered a full, complete, finished work of art.

The committee held a plenary session to discuss all aspects of the proposed legislation.

All of the people he interviewed for the job were plenary. This sentence works because the word means unqualified or absolute.