PCIe. In other words, Peripheral Component Interconnect Express. You may also hear the term PCI Express for short.
When your computer first boots, it has to figure out what components are attached to the motherboard. PCIe determines everything that is plugged in. It creates an internal map by identifying all the connections which allows the traffic to happen within the system.
PCIe connections are connected serially where bits of data are sent one at a time, not in parallel. The connection is made of one or more data-transmission lanes, each with one wire dedicated to receiving data and another dedicated to sending data.
Motherboards need to be able to cater to a large range of potential CPUs which each have their own requirements of how many PCIe lanes they will consume. Allocating the PCIe lanes efficiently determines how much performance you can get out of a motherboard. This can be difficult when industry standards require features leave less expansion slots to be allotted to. Attaching a RAID Controller on a motherboard that has NVME features leaves little room for more to be added.