The poet John Milton became blind later in his life, and this sonnet is one of the first that he wrote about his condition. The Biblical parable of the talents relates how a man who does not use his God-given talents while on earth will be punished, and Milton refers to it here because he is wondering how he will employ his talent as a poet to its fullest extent now that he is blind. Milton calls his gift "that one talent which is death to hide," but says it is "Lodg'd with me useless," now that is vision, or "light," is "spent." He would like to continue serving God with his gift, so that he will not be "chide[d]" or scolded when God calls him to account, but he wonders, "Does God exact day-labour, light denied," questioning whether God still expects him to work as he did before, now that he can no longer see. The Biblical parable is directly related to what the poet fears - that he will still be held accountable for using his gift as he did before, even though he is now blind.
Upon reflection, the poet finds consolation in the realization that God, who is almighty, does not need his paltry work. God is merciful and understanding, and Milton, now blind, can still serve him by being patient, and bearing his suffering without complaint - "who best bear his mild yoke, they serve him best." God's mercy transcends the literal message of the Biblical parable, and the poet is consoled with the realization that "They also serve who only stand and wait."