Copyleft is a way of sharing work — most often software — so that other people may use it, change it, and pass it on. There is a catch, and the catch is the point: they must give the next person the same freedoms. You cannot take a copyleft program, change it, and then lock your version up. Though actually, copyleft does not throw copyright away. It uses copyright law on purpose, almost backwards: the law that usually says “this is mine; keep off” is used to keep the work free. That flip is the joke in the name — copyright, but left. It is a tech and law word, not kitchen-table talk. People sometimes stretch it to music or writing with the same “share it the same way” rule, but software is still its home.