Z-buffering:
Z-buffering is a hardware routine that selects which polygons to draw and which to omit. Rasterizing polygons on-screen takes up a lot of time, it is therefore important that the CPU time is not taken up drawing polygons that cannot be seen by the user (i.e. they are off-screen or hidden behind other polygons that are either moving or closer). This routine speeds up the process of drawing polygons on-screen and permits a faster refresh rate.