If something is unmatched, there's nothing that quite compares to it. Your parents might buy a certain car because of its unmatched safety record.
Some basketball fans say that Michael Jordan's ability as a player is unmatched, while others argue that LeBron James has matched (and even exceeded) Jordan's status. And if the volume of your brother's burps is unmatched, it means he's the loudest ever. Another way to use this adjective is to describe one half of a pair, like a sock without its identical mate: "Hey, did you know your socks are unmatched?"