Someone who's ungrateful isn't thankful or appreciative. Your grandmother might think you're ungrateful if you don't send her a handwritten thank you note for the birthday gift she gave you.
You are ungrateful if you're unhappy with your neighbor's contribution to your yard sale — a big old television that doesn't work. Your neighbor, in turn, is ungrateful if he accepts the chocolate chip cookies you baked without even saying, "Thanks." The prefix un here means "not," and grateful comes from a now obsolete adjective, grate, "agreeable or thankful," from the Latin root gratus, "pleasing."