PCI Express is a high-speed serial computer expansion bus standard that is used to connect hardware devices to a computer's motherboard. It is a software-related specialized skill that involves knowledge of how to design, configure, and troubleshoot PCI Express devices, including graphics cards, network adapters, and storage controllers. This requires expertise in hardware integration, firmware development, device drivers, and system-level software. PCI Express is widely used in modern computers, servers, and embedded systems, providing high bandwidth and low latency connectivity between devices.