"Lift Every Voice and Sing" is a title that does a bunch of things at once. First of all, it's a title that calls for collective action: it's asking us, all of us, to lift our voices and sing. So the title is one that creates a kind of community, asking everyone to join in and lift their voices together.
The other important thing about this title is that it mentions singing. The word "sing" points to the fact that this is a poem that's also a song to be accompanied by music and performed out loud. It's a song-poem, in other words.
The reference to singing is also significant because it suggests the very important role that music has played in African-American history. Slaves used to sing on plantations as a way of getting through their difficult work. (That's how the blues started—why not sing if it helps pass the time?) Music and singing have been used by African-Americans for centuries not only to give expression to their experience, but also to protest their oppression.