The word "imperative" means "necessary" or "extremely important". It's a really intelligent- and academic-sounding word, which you might hear from teachers, in a business meeting, or in a blog post about economy or politics.
The grammar that comes with "imperative" is a little tricky. You say "It is imperative that", and then follow it with a clause that's in a specific tense like this:
It's imperative that I speak with him right away.
It's imperative that he understand what's at stake.
It's imperative that Russel not be allowed to take control of the committee.
It's imperative that the U.S. find a way to repay some of its foreign debt.
The verb is always in the present tense, and it doesn't change to agree with the subject.