"They also serve who only stand and wait."
This last line of the poem "On His Blindness" by John Milton relates to a person exercising patience and faith despite their circumstances. The person in this poem feels they are unworthy and useless to God. Their affliction- blindness - has hampered their ability to perform acts and deeds as they once did.
However, Patience lets this person know that in their affliction they can still serve God. In their affliction, God's great power becomes evident. God does not need the works of men or women. When we exercise humble acceptance of our circumstances and rely on God to sustain us, we inevitably become stronger - in the inward man - and allow God to work His pleasure in us.
The idea that Milton seems to be expressing in the sonnet "On His Blindness" is that God is responsible for everything that happens and therefore He is responsible for Milton's blindness. God does not require any work of him because of his handicap, but what God does require of him is that he submit to the handicap that He has decided to impose on him. This thought is assumed in the words "who best bear his mild yoke, they serve him best." Milton (undoubtedly the speaker in this sonnet) decides that he can best serve God by accepting his blindness and waiting for whatever else God has in store for him. He is not necessarily thinking that he is waiting for death or for the resurrection of Christ but only thinking that he is waiting for whatever God decides or has already decided will happen to him next. In a sense he means that he is waiting for further enlightenment, further understanding. It is another way of saying, "Thy will be done." No doubt Milton is also implying that by bearing his blindness he can set an example for others to follow.