A losing sente is, as it says, a sente play that loses points. Some other local play, normally a gote, but sometimes a sente with a smaller threat, is orthodox play. As a rule, playing a losing sente is wrong, but sometimes it is right.
For instance, you may want to make a losing ko threat. You may also gain more by keeping sente than by losing it and allowing the opponent to take the other side of the losing sente.