The PCI Express Architecture defines a flexible, scalable, high-speed, serial, point-to-point, hot pluggable/hot swappable interface for expansion cards such as graphics, audio, host adapters, SSD drives and other hardware. The PCI express bus connects to the northbridge making it extremely fast.
A PCI Express bus consists of one or more data-transmission lanes connected serially. Each lane consists of two pairs of wires, one pair for transmit and one for receive. There are 1, 4, 8 or 16 lanes in a single PCIe slot – written as x1, x4, x8, or x16.
x16 is usually used for high end graphics cards.
Read page 65 of the textbook “Exploring Computer Hardware”.