In “On His Blindness,” Milton expresses the worry that his loss of sight means that he will no longer be able to serve God. Milton was a deeply devout Christian, so this was a very important concern for him.

Although Milton wrote his best work after being struck down by blindness, he's still worried that there will someday come a time when he will no longer be able to serve God in his capacity as a writer. Milton draws upon the Parable of the Talents in Matthew 25 to illustrate his point. His is comparing the loss of his talents through blindness to the servant in the parable who buried his money in the desert instead of investing it.

As it turns out, Milton need not have worried. The personified figure of Patience assures him that God doesn't need “man's work or his own gifts”. Those who serve God best are those who patiently bear their sufferings. This is a mild rebuke to Milton for worrying over the potentially negative impact of his blindness on his ability to serve God.

As Patience explains in the last line, “They also serve who only stand and wait.” In other words, it is not necessary to serve God by doing anything, but simply by patiently bearing one's misfortunes. There is a place in the world for all of us, even those of us who cannot “post,” or move quickly, over land and sea.

How does this sonnet apply to us? It further suggests that each of us is given one or several talents which we are obliged to identify, utilize, and develop throughout our lives or else experience disappointment, frustration, and failure. The Bhagavad-Gita says something similar:

In the beginning
The Lord of beings
Created all men,
To each his duty.
"Do this," He said,
"And you shall prosper."   III Karma Yoga

The problem for many of us is to discover our talent, or talents. Doing this may involve a lot of trial and error. But it is obviously a matter of the utmost importance.

Blessed is he who has found his work; let him ask no other blessedness.
                                                   Thomas Carlyle

A man is relieved and gay when he has put his heart into his work and done his best; but what he has said or done otherwise, shall give him no peace. It is a deliverance which does not deliver. In the attempt his genius deserts him; no muse befriends; no invention, no hope.                                       Ralph Waldo Emerson

It seems to me that most people are good at what they like and like what they are good at.

John Milton's poem, "On His Blindness," speaks to the frustrations Milton had regarding his lost sight. The poem reflects upon the idea that he (the speaker of the poem) will not be able to serve God now that his sight is gone. The following will show each line of the poem (or relevant groups of lines) and the meaning of the line/s following.

WHEN I consider how my light is spent,

The speaker is reflecting upon how his light (sight) has been used over his life. This could also refer to the speaker's spiritual light (given Milton's religious ideology).

Ere half my days in this dark world and wide,

The speaker has spent half of his life blind (meaning he knows what it means to see and feels loss at his sight being taken away).

And that one talent which is death to hide

Here, the talent the speaker refers to can be his ability to write (which may be lost now that his sight is gone), or it could refer to the God-given talents bestowed upon mankind by God.

Lodged with me useless, though my soul more bent

The speaker feels that his loss of sight has left him useless (to either write or serve God).

To serve therewith my Maker, and present

My true account, lest He returning chide,
'Doth God exact day-labour, light denied?'

There three lines refer to the speaker's desire to write and praise God, but, with his lost sight, he feels as if he cannot do either. (The speaker's writing could be referring to his desire to write for God.) The speaker is asking if God expects him to work given his light (sight) is gone.

I fondly ask.

Here, the speaker is asking for guidance in regards to how he should approach his concerns (his inability to write and write to praise God).

But patience, to prevent
That murmur, soon replies, 'God doth not need
Either man's work or his own gifts. Who best
Bear his mild yoke, they serve him best. His state
Is kingly: thousands at his bidding speed,
And post o'er land and ocean without rest;
They also serve who only stand and wait.'

In these last lines, the speaker receives the reply he has asked for. Patience replies to the speaker (given his patience with his blindness is lacking). Patience replies that it is not the work of man which pleases God. Instead, it is the "mild yoke" (those who are simply obedient to God) which makes God happiest.

For patience, God is happiest when mankind is able to spread God's word over "land and ocean without rest." The final line refers to the fact that it is far more important to praise God than to "stand and wait" (do nothing).