Pcie risers x1 to x16 slot. Mostly used by crypto miners these days to use a lot of graphics cards in a single system. as for mining, a x1 interface is enough.
What @DigitalDude is correct, but also there is an surprising side benefit for these, You can build a JBOD case with these to power the HBA(not the "i" but the "e" type) card and skip the whole mb,cpu,ram thing.