OK principally means "adequate" or "acceptable" as a contrast to "bad".
It can also mean "mediocre" when used in contrast with "good". It fulfills a similar role as an adverb. As an interjection, it can denote compliance, or agreement. OK, as an adjective, can express acknowledgement without approval.
As a versatile discourse marker or back-channeling item, it can also be used with appropriate intonation to show doubt or to seek confirmation.