What is Tibet's surface like?
Tibet is located in the Himalayas that makes the surface mountainous, it is also a high plateau.
A concaved shape is one that is curved like the inner surface of a sphere. (Think about something that is cave-like.) The opposite, a convex surface, is one that is shaped like the outer surface of a sphere. So the mirror in a telescope has a concaved surface, as does the inside of a bowl and the surface of a crater or valley.