John Milton begins his poem by explaining his purpose in writing this poem. He writes:

Bitter constraint and sad occasion dear
Compels me to disturb your season due;
For Lycidas is dead, dead ere his prime,
Young Lycidas, and hath not left his peer.
Who would not sing for Lycidas? he knew
Himself to sing, and build the lofty rhyme.

The figure of Lycidas represents Edward King. Milton and King attended college together, and after King drowned, Milton was asked to write about him. Milton is speaking to the Muses when he opens the poem. He tells them that he is writing this elegy for King because he doesn't want the man's death to go unmourned. He says that King—or Lycidas, in this evocation—died before he even reached the best part of his life.

One image that Milton evokes throughout the poem is water. It's likely that he does this because of the manner of King's death. He even directly addresses the water nymphs to ask why they did not save the young man. He writes:

Where were ye, Nymphs, when the remorseless deep
Clos'd o'er the head of your lov'd Lycidas?
For neither were ye playing on the steep
Where your old bards, the famous Druids, lie,
Nor on the shaggy top of Mona high,
Nor yet where Deva spreads her wizard stream.

He appears to be accusing the nymphs for not saving a worthy man. He thinks that the water should have protected—rather than caused the death of—Lycidas. However, he goes on to recognize that life isn't fair, and even the lives of divine beings are filled with tragedy. He writes:

Had ye bin there'—for what could that have done?
What could the Muse herself that Orpheus bore,
The Muse herself, for her enchanting son,
Whom universal nature did lament,
When by the rout that made the hideous roar
His gory visage down the stream was sent,
Down the swift Hebrus to the Lesbian shore?

Even though Lycidas's death is painful and regrettable, ultimately Milton cannot find fault with the gods because of it. Milton repeatedly reaches out to divine beings in the poem, asking them questions about life and death.

Even though "Lycidas" is a sorrowful, elegiac poem to honor a man's life and mourn his death, there are glimmers of hope that emerge through its lines. For example, Milton writes:

Weep no more, woeful shepherds, weep no more,
For Lycidas, your sorrow, is not dead,
Sunk though he be beneath the wat'ry floor;
So sinks the day-star in the ocean bed,
And yet anon repairs his drooping head
And tricks his beams, and with new spangled ore
Flames in the forehead of the morning sky:
So Lycidas sunk low, but mounted high

Milton compares King's death to the setting of the sun every day. When one looks out at the ocean as the sun sets, it appears as though the sun sinks below the waves—a metaphorical evocation of the manner in which King died. However, every morning, it returns to its place in the sky as it rises. Milton says that Lycidas is the same. Even though he descended into the depths of the sea in his death, he will rise again.