PCIe x8 has half the number of PCI Express lanes as PCIe x16.
They can both use the same physical socket but for PCIe x8 only 8 PCIe lanes' contacts are soldered to the motherboard. PCIe x16 will have all 16 of the PCI Express lanes' contacts soldered to the motherboard.
Now if your talking about a PCIe x16 slot with all 16 lanes soldered to the motherboard and only working in x8 mode that means only 8 of the 16 lanes are active and the other 8 lanes have been disabled. You may also see this stated as PCIe x16 @ x8.