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

What exactly does the spec “PCIe x16 slot ” mean? pcie 3 0 x4 slot Making sure Graphics Card is running at pcie x16 speeds in bios & Windows pcie 40 in pcie 30 slot - 1 x PCIe x16 Slot , supports x4 mode* - 1 x PCIe x16 Slot , supports x1 mode*

pcie 3 slot The adapter supports PCIe x4 NVMe and PCIe-AHCI SSDs including 2242, 2260, 2280, and 22110 drives The expansion slot adapter supports the most