PCIe (PCI Express) is a high-speed serial bus standard that lets expansion cards communicate with your motherboard and CPU. It has replaced older parallel bus standards and remains the backbone of most desktop and laptop upgrades.
Every PCIe slot on a motherboard has a generation number (PCIe 3.0, 4.0, 5.0, and so on) and a lane count (×1, ×4, ×8, ×16). A slot marked PCIe 4.0 ×16 means it supports the latest generation at 16 lanes. Higher generations offer greater bandwidth, which matters most for graphics cards and NVMe storage drives. A graphics card uses a ×16 slot, whilst M.2 NVMe drives typically use ×4 lanes.
Real example: If you buy a modern RTX graphics card, it connects via the longest PCIe slot on your motherboard. An older motherboard with only PCIe 3.0 ×16 will still work, but newer PCIe 5.0 cards will operate at their full speed only if your motherboard supports PCIe 5.0.
When shopping for a graphics card or NVMe drive, check your motherboard's manual to see which PCIe generation and lane counts your slots support. A newer card will work in an older slot, but you may not get peak performance. For most buyers, PCIe 4.0 is now standard and sufficient for current gaming and productivity hardware.