In poetry, the speaker is the voice behind the poem—the person we imagine to be saying the thing out loud. It's important to note that the speaker is not the poet. Even if the poem is biographical, you should treat the speaker as a fictional creation, because the writer is choosing what to say about himself. Besides, even poets don't speak in poetry in their everyday lives—although it would be cool if they did.
In some poems, it's not even clear who the speaker is. But that doesn't mean we should ignore the poor guy or gal. Think of the speaker of a poem as a character—someone to be analyzed, thought about, and discovered. Where would Shmoop be if we didn't wonder who Prufrock really is?
Probably lingering in the chambers of the sea. But hey, that's just Shmoop.