Usually means when a reputed band of the alt. rock persuasion who sells themselves out to make a mainstream, top 40 hit song or album.
The definition was derived from the popstar Avril Lavigne's own hypocrisy and stupidity on her own career. When she first debuted she was touted as the Anti-pop Britney, singer. But as her career progressed she changed her image and persona into something which contradicted her own thoughts and actions.
I hope Gwen Stefani doesn't pull an Avril on the next No Doubt comeback...She already sold herself out with that Sweet Escape album bullshit.