These are the available lanes into the CPU from the PCIE. The data bandwidth is the base frequency (almost always 100 MHz) multiplied by the lane multiplier (x16,x8,etc..).
A newer GPU will run at PCIex16, and with some chipsets if you run multiple GPU's, the additional GPU's will run at x8, x6 or x4. Anything less than x16 is a bottleneck for a PCIE3.0 GPU.