What are PCIe Slots in PC?

What is the difference between PCIe Gen 3 and PCIe Gen 4? can you put a gen 4 ssd in a gen 3 slot PCIe is a much faster interface than SATA for connecting a host computer to solid-state storage devices over one or more lanes candy pop 2 slot On the it says PCIESATA does that mean I cant put gen 4 there? At the moment I have a WD PC SN530 PCIe Gen3 x4 NVMe installed on slot

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