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

What exactly does the spec “PCIe x16 slot ” mean? pcie 3 slot Riser with Three PCIe Gen3 slots 1 x16 FH 1 x8 FH and 1 x8 LP at least 2 processors R740XD CusKit The Riser from Dell™ fits into pcie x1 in x16 slot A PCIe slot will not bottleneck the card But what CPU are you running? That might be a different story 6162021 EVGA_JacobF Nope

pcie x1 in x16 slot List of Motherboards with 3 PCI-Express x16 Slots: GIGABYTE B650 Gaming X AX V2, MSI X670E Gaming Plus WiFi, ASUS ROG Strix Z790-E Gaming WiFi II,

pcie training error slot 1 3 Slot x16 Gen4 PCI Express Backplane Dolphin's IBP-G4X16-3 three slot backplane enables customers to establish a remote access to two PCIe devices  ✓ Mini PCIe to 3 PCIe 1x adapter expands 3 PCI express 1x slot via your motherboard with mini PCIe can use 3pcs PCI express card (Sound Card , Network