This refers to a special ability of someone on a team such as a Five-Man Band who is so specialized as to seem useless in most situations. Good writers can make this work for a character. They may have a power (such as it is), but it's not the real reason they're in the group. Some characters end up being The Heart (the social and moral compass), The Face (diplomat and spokesperson), The Smart Guy or the Badass Normal. Often said character is The Captain -- superpowers would be just a tiny bonus to his real ability: leadership. The Captain tends to be specifically written to have no superpowers, in fact.