Meaning of “imperative” - Learner’s Dictionary

imperative

adjective uk us /ɪmˈperətɪv/
MUST BE DONE formal

When an action or process is imperative, it is extremely important that it happens or is done:

[ + (that) ] It is imperative that I speak with him at once.
GRAMMAR

An imperative form of a verb is used to express an order. In the sentence 'Stop the machine!', the verb 'stop' is an imperative verb.

(Definition of “imperative adjective” from the Cambridge Learner’s Dictionary © Cambridge University Press)