The poem under consideration was first published by the poet in a collection called Milton's 1673 Poems. Almost 90 years later, publisher Thomas Newton assigned the title "On His Blindness" to the sonnet, which Milton gave only a number (19 in his notes, 16 in the self-published volume). Prior to that, "On His Blindness" was usually referred to by its opening line, "When I Consider How My Light Is Spent," but its subsequent title "On His Blindness" has been embraced over the last two and a half centuries.
In line 3, Milton refers to a "Talent" he can no longer apply toward labor he feels would be pleasing to God. This is an allusion to the parable of the talents in the Gospel of Matthew, chapter 25. In Jesus's lifetime, a talent was a weight of precious metal and therefore equivalent to money. In the parable, a man goes off on business, leaving different numbers of talents (that is, different values of money) to three servants. Two invest the money entrusted to them and earn profits for their master. The third merely buries the talent he was given, thereby earning no additional value. In Milton's poem, the speaker "buries his talent" by allowing a professional skill to go to waste. Unlike the servant in Jesus's illustration, however, the speaker has no choice but to do so. He's limited by a physical challenge— blindness (we should note that Milton's own blindness was already in full effect by the time the sonnet was published). Poetry is often assessed by how well it incorporates literary allusions and other forms of metaphor.
Milton's Gospel reference is a particularly evocative method of describing the speaker's physical limitations, casting them as an obstacle hindering his ability to please his God rather than merely an imposition to himself.
It is important to note that in line 8, the word "fondly" carries its now-outdated meaning of "unwisely," not the contemporary meaning of "affectionately." The speaker here asks God a foolish question: I want to please God, he insists, but will the Deity judge me harshly for being unable to use my full abilities? The reply, which the speaker hears as a whisper from personified "Patience" in his mind, reminds him that God is perfect unto Himself and requires no help whatsoever from human beings. He has the aid of thousands of angels at his beck and call. Thus, Patience reminds the speaker to serve God meekly, without worrying overmuch about the limits of mortal abilities. Patience's advice culminates in a now famous line: "They also serve who only stand and waite."
Some critics and commentators believe "On His Blindness" to be Milton's noblest sonnet. There is clearly an autobiographical element to the poem, and we empathize with Milton and his handicaps and resulting self-judgment. Most humans grow old. Our bodies grow infirm; we lose the vitality of youth and find ourselves unable to do some things we used to find important and pleasurable. It's easy to wonder if our value to the world around us has diminished, or even if God might be angrily punishing us for failing him somehow.
The universality of Milton's poem has endured for over three hundred years. It articulates his answer to the ever-present question of suffering. If God loves us, why does He allow our lives to be so difficult and short? Milton seems to imply that our struggles on earth are merely preparation to serve the Universal King most fully in the higher plane.