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

Fastest NVMe SSDs in 2024 m2 nvme slot By specification, all PCIe cards are backwards compatible with the lowest bandwidth x1 lane slots If the x1 slot is designed with an open end, m24 slot 2 slots With a PCIe NVMe + SATA dual-mode design, users can build a RAID storage system with two NVMe SSDs or a SATA Mode M

m2 slot 2 SSD? SATA vs M2 NVMe; B key, B+M key, and M key; Length A B key only works on a B slot, and the same hols for an M key and M slot