Something that's all the rage is the hottest, snazziest, chicest craze — nobody can get enough of it right now. But they probably won't love it in two weeks, because being all the rage is never permanent.
All the rage comes from a definition of rage that is no longer very common: a trend or fashion. Clothing styles, dances, songs, slang terms, collectibles, and more could be all the rage at a moment. Because trends rarely last very long, all the rage contains a strong suggestion that whatever is being discussed is not going to stick around for a very long time.