The PCI slot has one simple goal. It allows a user to add on custom pieces to work with their computer. Without such expansion slots, you'd have to order a very customer motherboard which wouldn't be very cost effective. Instead, you add on the components you want to customize your computer.
In the picture below, the PCI slot is number 8. The view below is the same board without the number.
There are 2 types of PCI, PCI and PCI-e or PCI express. When buying a card, beware the 2 are not interchangeable.
Adding a PCI card to a computer is a much simpler process than most
people think. All you need to do is shut off the computer, remove the
power wire, then after opening the computer case, you slide the card
into the slot, and fasten it with a screw. After your system
restarts, you might need to install a driver, usually included with
your card, after which time the card is ready and available for use.