I weep for Adonais—he is dead! Oh, weep for Adonais! though our tears Thaw not the frost which binds so dear a head! And thou, sad Hour, selected from all years To mourn our loss, rouse thy obscure compeers, And teach them thine own sorrow, say: "With me Died Adonais; till the Future dares Forget the Past, his fate and fame shall be An echo and a light unto eternity!
Welp, it looks like someone named "Adonais" has bit the dust, and the speaker is mourning his loss.
In Greek mythology, Adonais (also spelled "Adonis") was a beautiful youth who died young.
But wait, isn't this a poem about John Keats, the poet? Yep. By referring to Keats as "Adonais," Shelley is using the god as a symbol of the poet. It's his way of celebrating Keats' beauty (or the beauty of his words) and mourning his early death.
You see, Keats died at age 25, which even in the olden days was considered pretty darn young. This was devastating to Shelley, so he decided to write a poem (er, this poem) in his honor. So, from here on out, Adonais = Keats (and vice-versa).
The speaker then calls on us to mourn (to "weep for") the loss of the youth. He admits that it won't do much good, though. It won't "Thaw […] the frost which binds so dear a head!" (3). Using a metaphor, the speaker compares death to being frozen. Here, too, the speaker uses a synecdoche when he uses the frozen (in death) "head" to represent Adonais-Keats as a whole.
Still, he wants us to "rouse" (wake) others ("obscure compeers") and make them share in the sorrow.
The speaker wants the youth's legacy to live on, his fame an "echo" and a "light," both of which can be made to continue forever. So, yeah, it's safe to say the speaker is pretty devastated about this loss.
He still manages to rhyme, though. (So at least there's that.) As you follow through the poem, keep your eye on the wordplay and rhythm in his lines, and refer to our "Form and Meter" section for the inside scoop.