Wow, OK...
The expansion slots on your motherboard, those deals where you can plug in a sound card, or network card or whatever...back in the day they used to be PCI (actually before that was the ISA bus but I digress) PCIe stands for "Peripheral Component Interconnect Express" and actually what you are checking for are more specifically PCIe x16 slots that will support video cards.
They look like this
but the plastic part doesn't have to be black, it can be any color.
While you have the cover off also measure how much room you have between the slots in the back of the case where the cards screw in and your drive bays, these cards are almost a foot long. Also, be careful in there. Shut the computer off and unplug it first and don't touch shit you don't have to.
