Stands for "Peripheral Component Interconnect." It is a hardware bus designed by Intel and used in both PCs and Macs. Most add-on cards such as SCSI, Firewire, and USB controllers, use a PCI connection. Some graphics cards use PCI, but most new graphics cards connect to the AGP slot.
PCI slots are found in the back of your computer and are about 3.5" long and about 0.5" high. So before you go buy that Firewire expansion card, make sure you have at least one PCI slot available.