Okay (also spelled OK, O.K.) is a colloquial English word denoting approval, assent, or acknowledgement, but it can have several different meanings.

(As an adjective, “okay” means “adequate,” “acceptable” (“this is okay to send out”), but it can also mean “mediocre” often in contrast to “good” (“the food was okay”).

As an interjection, it can denote compliance, “Okay, I will do that”, or agreement, “Okay, that’s good”.

Often it does not modify any other particular word, but rather reinforces the general point being made, particularly if that point is being called into question. For example, a response to “So the accident kept him from going to the reunion?” might be “Oh, he went to it okay, but he had bruised ribs and his car was a wreck.” In this case “okay” does not modify him or his going anywhere; it is a particle emphasizing the point that is being questioned.
As a noun and verb it means “assent”, “The boss okayed the purchase”.