What does 1 Samuel 16:22 mean?

Due to his disobedience, Saul had been rejected by God as Israel's king (1 Samuel 13:11–14; 15:17–29). Though Saul would retain the throne for at least ten more years, God removed His Spirit from Saul (1 Samuel 16:14). He also allowed an evil spirit to torment the sitting king. Aware of the evil spirit's presence, Saul's servants recommended he hire a musician to soothe him (1 Samuel 16:15–16). Saul agreed. A servant recommended David, and David has entered into Saul's service (1 Samuel 16:17–21). Saul has quickly grown attached to the young man, even making him his armor-bearer, one of the most relied upon positions any servant could hold in the kingdom. The previous verse reported that Saul loved David (1 Samuel 16:21).

Now Saul sends word to David's father, Jesse, letting him know that David will stay on as his servant in a permanent position. Apparently, Saul's original expectation was that David's service as a musician would be temporary. David, though, quickly became one of Saul's closest and most trusted servants.