In this sonnet, Milton laments that his blindness prevents him from using his chief gift, that of being a talented and very knowledgeable writer, to serve God. He emphasizes his despair by focusing in these opening lines on the negative: his light is "spent" or used up before his life is half over. The word "dark" emphasizes his despair. The world is dark because he is blind and thus can't see anything, but the world, in his religious perspective, is also dark because it has fallen into sin. He is a blind man in a sinful world and is unable anymore to use his talent—his "light"—to lead the world from darkness to light. The lines convey his frustration at his sense of uselessness. He is not ready to give up his vocation, for his life is not over yet. But by the end of the sonnet he comes to terms with his situation, and in a famous line says:

They also serve who only stand and wait.

Sonnet XIX by John Milton, often called "On His Blindness" begins:

When I consider how my light is spent

Ere half my days in this dark world and wide ...

These lines introduce the poem as an autobiographical one reflecting on the poet's having gone blind in early middle age. Before writing this poem, Milton had a distinguished career as writer and  translator. He was also deeply religious and considered that his writing was a form of service to God and his agonizing over how he can no longer perform this service now that he is blind. 

His light thus refers both to his own work, illuminating the ways of God for man and also his life. By describing his life as "spent", he is emphasizing not only spending his light in terms of how he passed his days but also spending it in the sense of having a fixed amount that when "spent" of gone. Thus these lines emphasize that his years lived are years that are gone and that he cannot get back, emphasizing that his life is almost half over and that each day he spends brings him one day closer to death. The notion of light being "spent" also suggests his progress into the total darkness of blindness.