It is the support circuitry on the PC motherboard.

These are the little pieces of silicon that hold computer information and instructions. Just about any computer component has at least one chipset on it. Motherboard chipsets control the basic ins/and outs of the computer. Video card chipsets control the rendering of 3D graphics and the output of images to your monitor. The CPU is just a very important chip. Common motherboard chipsets include the BX, i810, i820, and many others.