It’s a chunk of metal and two bolts that allow you to mount different-sized disc rotors on your bike frame. Basically, it’s a spacer. Bigger spacers = bigger rotors. Not all disc brakes need adaptors: If a bike frame is designed for 160mm rotors and post-mount brakes, and you stick to that size of the rotor, no adaptor is needed. That’s pretty much only the case with Post Mount.