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

What exactly does the spec “PCIe x16 slot ” mean? x4 slot They include hot-swappable SATA 6 Gbps HDDSSD, dual RJ45 LAN ports, a PCIe Gen 3 x4 slot, and two USB Gen 1 (5 x4 slot PCIe x4: This slot has four lanes for data transfer It provides more bandwidth than an x1 slot and is commonly used for various expansion cards, including

x4 slot PCIe boards can fit into slots designed for their lane configuration or higher Plugging a x4 PCIe into a x16 slot is acceptable

slot canyon Xiaomi Poco X4 GT Specifications Xiaomi Poco X4 GT MORE PICTURES Card slot, No Internal, 128GB 8GB RAM, 256GB 8GB RAM UFS Main Camera  List of Motherboards with 1 PCI-Express x4 Slots: ASUS ROG MAXIMUS Z790 HERO BTF, ASUS TUF Gaming Z790-BTF WiFi, ASUS ROG Maximus Z790 Formula,