What does the X4 mean in an PCIe Gen X4 NVME drive?

PCIe adapter with four slots - Storage pcie 3 0 x4 slot PCIe slot will run at the lowest denomination speed This particular motherboard has 4 PCIe x16 slots, 1 PCIe x4, and 1 PCIe x1 pciex1 slot Hello community, I just bought a used Z2 Mini G3 Workstation and added a HP Z Turbo Drive G2 SSD to the slot on the

pcie4 slot SuperSpeed Storage：PCIe x4 Connector; EZ-Latch：PCIe x16 Slot with Quick Release Design; Fast Networks：GbE LAN; Extended Connectivity：DP, HDMI; Smart