Mesh
A 3D surface built from many small triangles that share their corners.
See it in Depth terrain. Try a sample map, then set Render mode to Wireframe.
In plain words
A computer draws 3D shapes out of flat triangles. A mesh is a big set of them joined edge to edge. Each corner is a point with a position in space, shared by the triangles that meet there. With enough small triangles, even a round hill looks smooth. The more triangles, the finer the shape, and the more work to draw it.