UV mapping
Pinning a flat picture onto a 3D surface by giving each corner a spot on the image.
See it in Depth terrain. Try a sample map, then set Render mode to Textured.
In plain words
A 3D surface is made of triangles, but pictures are flat. UV mapping tells each corner of each triangle which spot of the picture it should show. U counts across the picture and V counts up or down it, so they do not mix with x and y. The GPU then stretches the picture across every triangle. Think of wrapping a present with printed paper.