If it is imperative that something is done, that thing is extremely important and must be done.

An imperative is something that is extremely important and must be done.

In grammar, a clause that is in the imperative, or in the imperative mood, contains the base form of a verb and usually has no subject. Examples are 'Go away' and 'Please be careful'. Clauses of this kind are typically used to tell someone to do something.

An imperative is a verb in the base form that is used, usually without a subject, in an imperative clause.