What exactly does the spec “PCIe x16 slot ” mean?

What exactly does the spec “PCIe x16 slot ” mean? pcie4 slot 20x full-size card slots · 4x PCIe x16 Gen from CPU · PCIe x16 Gen to each slot · Active air flow from edge to bracket · Power budget: hydras lair slot Alienware's laptops actually limit the slot to x8 PCIe , siphoning off the rest to support the external graphics port The reason? It doesn't

hydras lair slot Supported by Xgig tool suite including Trace Control, Expert and Serialytics VIAVI Solutions Xgig® 4-lane, CEM-slot Interposer, the 4K-PCIe4-X4-AI, provides

gameplay slot You'll be able to do more with less in this case, since a PCIe x8 slot can handle almost exactly what a PCIe x16 slot can currently x slot, or a card into a slot and in theory everything Is there much to gain from having a pcie 4 board and pcie 4 GPU?