PCIe slots, or Peripheral Component Interconnect Express slots, are interfaces on a computer's motherboard that allow for the connection of various hardware components. These slots enable devices like graphics cards, network cards, and storage controllers to communicate with the computer's processor and memory, enhancing overall performance and functionality. There are different sizes of PCIe slots, including x1, x4, x8, and x16, which refer to the number of data lanes available for communication. The more lanes a slot has, the higher the potential data transfer speed. This flexibility allows users to upgrade their systems easily by adding or replacing components as needed.