Main Point
Essentials
noun
The parts we need. The point of that. That is the main point. That is the essentials. It's what we need and it's to the point, you know?
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
授業前にその本の要旨を理解しておくと、プラスになると思うよ。
I think it would be a plus to understand the fundamentals of the book before the lesson.