All CPUs have a limited number of PCIe 'Lanes'
Read more details here:
https://en.wikipedia.org/wiki/PCI_Express#LaneLets say a CPU can handle up to 28 lanes
If you plug in one graphics cards directly to the motherboard, it takes 16 lanes.
If you plug the graphics card into a 1x riser, then it only takes 1 lane instead of 16.
This means your CPU will have 27 lanes leftover instead of 12.
Other devices use these lanes as well, like M.2 SSD.
Here is another good explanation.
https://superuser.com/questions/843344/what-is-a-pci-express-lane