A rocker can mean several things:

1. A rocking chair, the kind that moves back and forth when you sit in it. Grandparents often have rockers on their porches where they can sit and read or watch the world go by. The curved pieces on the bottom, called runners, let the chair rock smoothly forward and back. Babies often fall asleep in rocking chairs because the gentle motion is so soothing.
2. A musician who plays rock music, especially as their main profession. When people say someone is a rocker, they usually mean this person performs rock and roll: loud, energetic music with electric guitars and drums. Famous rockers like Bruce Springsteen or Joan Jett built their careers around this powerful, rebellious style of music.
3. The curved piece on the bottom of a rocking chair or rocking horse that makes the rocking motion possible. Without the rockers, it would just be a regular chair.

When someone says a person is “off their rocker,” they mean the person is acting crazy or irrational, like a rocking chair that's lost its balance and tipped over.