A PCIe x16 (x4) slot is a PCIe x4 slot, designed so that PCIe x16 card can also be inserted into the slot an work, albeit running at the slower speed of PCIe x4. This type of slots allows more solid state drives, for example, to be inserted than the number of actual optimally-sized connections (x4 for solid state drives) the motherboard or CPU can offer.
This slot in a not-very-common shape explains the concept well: