Signature Moves are moves that can only be used by one person. They cannot be used by anybody else and are usually super powerful, with a power level of around 6800 or higher. Many people have signature moves, but not everybody does.
Signature Moves are obtained when one teaches themself a powerful move, or create the move by testing different attack motions with different types of energy in hope of creating something powerful.
Signature Moves are only signature moves if nobody else can possibly use them, no matter what in the Known Dimension they do, so they are really really hard to learn. They also cannot be taught to another person (If it could be taught, then it wouldn't be a Signature Move), as the other person would not be able to learn it anyway.