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, and network cards, enhancing the computer's capabilities. Common types of expansion slots include PCI, PCIe, and AGP. Each type has different specifications and speeds, catering to various hardware needs. By utilizing expansion slots, users can upgrade their systems without replacing the entire motherboard, making it easier to keep up with technological advancements.