The speaker starts us off by saying that he knows exactly what a caged bird feels. It doesn't feel good, especially when outside of the bird's cage the sun is bright and a river is flowing and the wind is stirring through the grass. The bird can't go out and frolic in that beautiful landscape. Not only that, but—stuck in its little cage—this bird can't hang out with other birdies, and it can't smell the beautiful flowers. At the end of the first stanza, the speaker again tells us that he knows how the caged bird feels. In other words, he understands why it suffers from being locked up in a cage.
In the next stanza, things get a little bit more violent. The speaker tells us that he knows why the caged bird "beats his wing" until they're bloodied against the bars of its cage. Yikes—this birdie is really miserable. This stanza focuses on the idea of physical pain. Not only is the bird wounded because it beats its wings against the cage, trying to get free, but every time it does so its "old scars" throb again with a new pain.
In the final stanza, the speaker tells us he knows why the caged bird sings. We can guess by this point in the poem that this bird is singing not because it's a happy bird. After all, its wing is "bruised" and its bosom is "sore." The caged bird's song, the speaker tells us, is a prayer and a plea that he (the bird) sends to heaven. Though the speaker doesn't tell us what this prayer is for exactly, we can assume that it's a prayer and a plea for freedom.