In real-time polygon rendering
engines, it's a way of using more than one texture map
in a single pass
Say you want to apply two textures to a particular polygonal surface to render a scene -- for example (and this is the most common case), one is the underlying texture of the object and the other is a lightmap. Usually, to do this, you have to render the polygon twice: once for each texture -- a so-called multipass approach. However, recently 3D graphics hardware (such as the nVidia TNT series) have allowed the application of more than one texture at a time. The original cards supported two textures simultaneously, but newer cards such as the GeForce 3 support up to four textures in a single pass.
This saves you a lot of time. If every polygon is dual textured, for example, the scene could render in half the time.