A toponym is the name of a place. Boston, Australia, and Montreal are all toponyms.
Words beginning with topo usually have to do with places, like a topological map. Words ending in nym are types of names, like an eponym is a name for a thing that came from a person's name. A toponym, therefore, is a name for a place. Wherever you live, its name is a toponym: United States, North America, Atlanta, and California are all toponyms. Even names of made-up places like Narnia and Atlantis are toponyms.