Left over and leftover are two version of the same term, which means remaining or not yet used as an adjective. Left over comes after verbs in sentences. Leftover comes before the noun it modifies. Leftover can also be a noun, where it refers to food that has not been eaten.

From Longman Dictionary of Contemporary English left‧o‧ver1 /ˈleftəʊvə $ -oʊvər/ adjective [only before noun] remaining after all the rest has been used, taken, or eaten leftover vegetables a few pieces of leftover carpet Examples from the Corpus leftover • Take all your leftover bottles to be re...