Something that's multicellular is a complex organism, made up of many cells. Humans are multicellular.
While single-celled organisms can't usually be seen without a microscope, you can see most multicelluar organisms with the naked eye. Plants and animals are multicellular — although they all, including humans, start as a single cell that reproduces into many. Multi means "many," and the Latin root of cellular is cellularis, "of little cells," from cella, "small room."