Expansion slots are connectors on a computer's motherboard that allow users to add additional hardware components. These slots enable the installation of various expansion cards, such as graphics cards, sound cards, or network cards, enhancing the computer's capabilities. Common types of expansion slots include PCI, PCIe, and AGP. Each type has a specific design and speed, catering to different hardware needs. By utilizing expansion slots, users can customize their systems to improve performance or add new features without replacing the entire motherboard.