Multicellular organisms are made of more than one cell ("multi" = many). Unicellular organisms are made of only one cell ("uni" = one).
There are six kingdoms within the classification of organisms (the branch of science known as taxonomy). The six kingdoms are: bacteria, archaea, protists, fungi, plants, and animals. (The five kingdom system of classification is considered to be largely outdated).
The two kingdoms that are considered to be made entirely of multicellular organisms are plants and animals. Although most fungi are multicellular, yeasts are a type of fungi that is unicellular.