Key(Noun) = a small piece of shaped metal with incisions cut to fit the wards of a particular lock, which is inserted into a lock and turned to open or close it. It is similar to the word 'Important'

Synonym: Significant, Worthy, Important

Antonym: Trivial, Insignificant, Unworthy

Sample sentence:  When he reached her door, he turned the spare key in the lock and walked inside.