Texturing is the ability to place an image ontop of geometry. It adds greatly to the realism of the scene. Here is a simple example of texturing.
Texturing usually involves three steps. One is telling the graphics api about the image. The second is telling the graphics api which image you want to use and how you want to use it on a given piece of geometry. The third is to send the geometry to the graphics api with the appropriate set of texture coordinates.