He lives, he wakes—'tis Death is dead, not he; Mourn not for Adonais. Thou young Dawn, Turn all thy dew to splendour, for from thee The spirit thou lamentest is not gone; Ye caverns and ye forests, cease to moan! Cease, ye faint flowers and fountains, and thou Air, Which like a mourning veil thy scarf hadst thrown O'er the abandon'd Earth, now leave it bare Even to the joyous stars which smile on its despair!
All those that the speaker called on to mourn earlier in the poem are now called on to stop their mourning.
The speaker starts with nature, and gets pretty specific about their (personified) mourning.
He wants the caves and the forests to stop moaning in grief, he wants the flowers and fountains to come back to life, and he wants the air to stop fogging up the earth, like it has been covered in a mourning veil (simile alert).
Even the stars should be allowed to smile down on earth, though they too despair over the youth's death.