PCIe slots are found on all modern motherboards, and come in four different types: x1, x4, x8 and x16. The number that is listed next to the slot indicates the number of PCIe lanes that can be used.
For example, a x1 slot has one PCIe lane, while a x16 slot has 16 PCIe lanes.
x1 slots are the most common type of slot and can use one PCIe lane. x4 slots can use two PCIe lanes and are great for adding extra graphics cards or other expansion devices.
x8 slots can use four PCIe lanes and are perfect for adding two high-speed storage devices or two additional graphics cards. Finally, x16 slots can use eight PCIe lanes and are great for adding multiple high-speed storage devices or multiple graphics cards.
When choosing a motherboard, be sure to check which type of slot is on the board and how many PCIe lanes it supports. Some boards have more than one type of slot, so be sure to find the right one for your needs.