I hope you are talking about the PCI of COA ie Computer Organisation Architecture. I'll answer you with that respect, if am wrong, feel free to mention in the comment.

So generally PCI stands for Stands for "Peripheral Component Interconnect." PCI is a hardware bus used for adding internal components into desktop computer. For example, a PCI card can be inserted into a PCI slot on a motherboard providing additional I/O ports in the computer.

PCI is an interconnection system between microprocessor and additionally connected devices. We can basically use PCI when we need to connect additional hardware to PC other than that are available on motherboard.

and PCI slots are designed to connect hardware to system or PC.

These are basically used for graphics card, Network interface card etc.

Thank you.