It´s said that one is singing in playback, as opposed to "singing live". The singer is miming at some point or miming all over the music.
The mag Pop Crush
asked in an interview about that to Britney Spears, about the playback singing issue, and she answered:
Interviewer: How much of your show is with playback — backed up with playback, how much of it is live?
Britney: "I'm glad you're addressing this question because it's really funny. A lot of people think that I don't sing live. I do usually — because I'm dancing so much — I do have a little bit of playback, but there's a mixture of my voice and the playback.
Of course, the singer may have some problem with his voice, nobody has the voice good all the time, so the show goes on.
Sometimes, though, something happens that exposes that the singer was lip-syncing, to singing to playback song! Like this guy that was helping Lady Gaga and accidentally ended up exposing her:https://www.youtube.com/watch?v=u7RKNy7If60