The saying "a piece of cake" means something that's simple to accomplish. If a school assignment is a piece of cake, it's so easy that you will barely have to think about it.
Other ways to say "it's a piece of cake" include no problem or it's a breeze. Something that's a piece of cake is as easy as eating a delicious piece of cake would be. The Americanism cakewalk, used to mean "something easy," came first, in the 1860's — piece of cake wasn't used until around 1936. Both cake and pie have a long history in the United States as metaphors for things that come easily.