Basically, the user buys a backplane with a number of PCI Express slots, which can be populated to suit the user's needs. This "motherboard" no longer has a CPU socket, all of the computing functionality is now provided by a replaceable host PCIe card.
Intel showed off an "Element" at the show that featured a Xeon CPU (a soldered BGA model), DRAM, M.2 storage, and I/O on a single dual-slot PCIe card.
Ultimately with the Element, Intel wants to make it easier for integrated system upgrades. Customers can keep the chassis, keep the system setup, keep the backplane, and all they would do is change the Element card to get the latest performance and features. This was the ultimate goal with something like Razer’s Project Christine, and is certainly something to work towards. However, by keeping the storage on the Element rather than having it as a separate add-in card, this is somewhat limiting as it would require swapping the drives out. This might not be much of an issue, if one of the PCIe slots on the backplane was used for M.2 drives (or even with drives on the backplane itself).It's an interesting concept but whether it will gain mainstream traction remains to be seen. The momentum to continue to use ATX is very strong, but there may be a niche market for this, similar to what we've seen with the NUC. According to Intel, the first "Element" based systems may hit the market in Q1 2020. Full details at AnandTech.