A rocker is a chair that can move gently forward and backward, or a rocker is someone who appreciates rock and roll music. You could be a rocker who rocks your rocker to the beat of Guns N' Roses. Rock on!
You can also call a rocker a rocking chair. When a person is a rocker, they might be "someone who rocks in a rocking chair," but it's more likely that they're in a rock and roll band. Some rockers don't perform rock music, but they love it: "My dad was a total punk rocker back in the 80s." The chair meaning is from 18803 American English, while the rock and roll meaning originated in the 1960s.