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.

But most notably. It happens to be the most popular word in the world.