It’s a piece of software which, when given the specifics of an onscreen area, fills in that area with a specified texture and/or shaded light pattern.
The onscreen area is almost always polygonal and usually triangular, and its virtual angle with respect to the viewer
and the light source(s) are generally specified as well. It’s used to texture, color, and shade 2D areas which represent 3D objects in a convincing manner,
and is usually highly optimized so as to do its job as quickly as possible (very important,
since it may render an entire screenful of pixels more than thirty times per second).