This article was created from a community discussion by @JustinW.
PCIe bifurcation is a feature that allows the division of data lanes in a PCIe slot. For instance, on supported systems a PCIe x16 slot can be configured into two PCIe x8 lanes, or more smaller lanes, depending on what the motherboard supports. PCIe bifurcation does not affect overall speed but does allow a larger slot to act like multiple smaller slots, allowing the use of some advanced expansion cards without additional PCIe bridge chips.
This feature can also enable the use of multi-drive NVMe M.2 expansion cards, as each drive needs its own PCIe x4 lane to work correctly. A PCIe x8 to dual NVMe M.2 card typically requires the ability to access two separate PCIe x4 lanes, one for each drive. Normally this can be done via a x8 card with a special PCIe chip that handles this split, but generally results in greater complexity and higher cost for the card. The alternative is a PCIe x8 card that doesn't have the bridge chip but can make use of PCIe Bifurcation natively on the motherboard.
Not all motherboards or BIOS/UEFI versions support PCIe Bifurcation. Please refer to the documentation for the specific motherboard/system to see if PCIe Bifurcation is supported and, if so, how it can be configured.