in VERY simple terms: (coz I am a simpleton)
think of how current 3D card produce a scene in Quake3, lets say, a rocket shooting from Point A to Point B.
The renders graphic information like the front side of the rocket, the sides, the back, the flame, the smoke, and the explosion. while it process all these information out, the monitor can only show part of it, (coz, you only see back side of the rocket unless you are the one who's getting shot 🙂, in which case, you will see the front if you stood there like an idiot.)
Hidden Surface Removal basically take away those information/stuff we don't see, and hence reduce the load of the card and improve performance.
the pioneer of such technology, to my knowledge, is PowerVR's Kyro3, which is known as Tile Based Rendering instead. ATI's is known as HyperZ
