What is a large area of nearly flat land?
A large area of nearly flat land is called a plateau. These plateaus can be covered grasslands or made of rock.
Geographically, a "plain" is a large, treeless area of level or nearly level land. By convention the term "plain" is usually used to describe a low level flat area such as a coastal plain, sea floor plain, a river plain or a lunar plain (mare) rather than an elevated flat area which would be called a plateau.