AGP (Accelerated Graphics Port) is a bus system mainly used to connect graphics cards to the rest of the system.
If you have an AGP system and you say Y here, it will be possible to use the AGP features of your 3D rendering video card. This code acts as a sort of "AGP driver" for the motherboard's chipset.
If you need more texture memory than you can get with the AGP GART (theoretically up to 256 MB, but in practice usually 64 or 128 MB due to kernel allocation issues), you could use PCI accesses and have up to a couple gigs of texture space.
Note that this is the only means to have XFree4/GLX use write-combining with MTRR support on the AGP bus. Without it, OpenGL direct rendering will be a lot slower but still faster than PIO.
You should say Y here if you use XFree86 3.3.6 or 4.x and want to use GLX or DRI. If unsure, say N.
This driver is available as a module. If you want to compile it as a module, say M here and read Documentation/modules.txt. The module will be called agpgart.o.