In the octave of the sonnet "On His Blindness," Milton laments that being blind prevents him from serving God as effectively as he might otherwise have done. In the sestet, a personified figure of patience replies to him, pointing out that God is a better judge of such matters than he is. God is constantly sending thousands of people around the world to do his work. However, "they also serve who only stand and wait."
This last line of the poem makes an obvious point which applies to God, as it would to any temporal king or master. If you were employed in the service of a king and, one day, he gave you nothing to do, you would still be that king's servant. If anyone were to ask whom you served, you would still say the name of the king, despite the fact that you happened not to be doing anything for him at that moment.
Patience tells Milton that he is already serving God by being willing and ready to serve him. God will find the right employment for his hands at some point. In the meantime, however, he need not be too officious about looking for work or complaining that he cannot exercise his abilities in God's service.