When something happens overnight, it occurs during the hours of one night. An overnight party can also be called a sleepover or a slumber party. (Pack your sleeping bag, but you probably won't use it.)
You can use overnight as an adverb or adjective literally meaning "during the night" — like an overnight airplane flight or a guest who stays overnight on your couch. You can also use it figuratively, to mean "quickly" or "instant." After you publish your first book, you may be an overnight success; and after an important election, the world may seem to change overnight.