John Milton's "On His Blindness" is written in the form of a Petrarchan sonnet, consisting of an eight-line octave rhymed ABBA ABBA and a sestet rhymed CDECDE. As is typical of the genre, the poem poses a problem in the octave which is then resolved in the sestet.
The main theme of the poem is autobiographical. Milton, born in 1608, had been an active writer and engaged in the main political struggles of his period in his writing for most of his career. In 1654, he went blind. This blindness, occurring in middle rather than old age, is what he reflects upon in the lines:
WHEN I consider how my light is spent
E're half my days, ...
The rest of the octave is a complaint that he used his writings to serve God, but being blind, he is now limited in his ability to perform that task, as he must be read to rather than being able to read, and dictate his works to assistants rather than being able to write himself. He wonders why God is depriving him of the very tools he uses to serve God. That complaint is then resolved in the sestet by acceptance of God's will.