Usually it's a car that has been totaled out and given a title so you can rebuild it. Insurance company let them "buy it back" for a really low price, like $1000. Then they replaced all the damaged parts. It's sometimes worth it to rebuild a car, but unless it's dirt cheap, DON'T buy one with a rebuilt (aka: salvage) title.
You may have trouble getting insurance for it, and you have no idea if any repair were done right. The frame could be fukced, since some rebuild inspections ONLY check if it's roadworthy and the parts replaced to insure you didn't used stolen parts on them.
