Doth God exact day-labour, light denied?
'day-labour' is a reference to work performed during daylight hours. Since it was customary, and generally still is, to work during the day, Milton is asking whether God still demands of him to work, even though he is blind. God has blessed him with a particular talent (his ability to write), and he therefore wishes to know that, since he is so blessed, whether God still expects him to utilise this ability in spite of his disability.
Milton provides the answer in the following lines:
"God doth not need
Either man's work or his own gifts: who best
Bear his mild yoke, they serve him best.
In these lines, Milton expresses his unfailing belief in God's righteousness. God is not dependent on man, but man is reliant on Him. God does not expect man to do anything but 'bear his mild yoke'.
'mild yoke' is a reference to the very slight burden that man has to bear. Serving God is not that great a task and therefore the burden - 'yoke' is not such a strain. The word 'yoke' is a reference to a wooden crossbar carried on the shoulders by a farmer or labourer or even a large farm animal. This is usually attached to a plough which is drawn through soil to cut into grooves in preparation for sowing.
God therefore does not expect man to bear too great a burden. It is more than enough, as Milton puts it,
They also serve who only stand and wait."
Milton earlier asserts God's kingly status and his great generosity, He is appeased by those of his servants who 'stand and wait'. In this instance, Milton is saying that God does not expect man to perform grandiose deeds to benefit from his kindness and receive redemption, but just as a soldier, who patiently stands guard, is performing as much of a duty as an active one, do those subjects who wait for God's grace and His Second Coming, perform a task. They bear a 'mild yoke' and need only wait for Him to benefit from His grace.