What is this attaching my toilet seat and how can I unscrew it?
That is called a blind jack nut, jack nut, blind collapsing nut, etc.



They are typically used when you don't have access to and/or don't have the tools to hold onto a nut on the backside of whatever you're putting it into. As you turn the bolt the wedge will move along its length, moving into/out of the split nut which adds/removes force that pushes the split nut apart. In the non installed position the wedge is at the end of the bolt so there is nothing in the split nut to force it apart &amp; the entire assembly can be pushed/pulled through a hole.

In your case the bolt is accessed from the opposite side. There are usually caps covering the bolt that can be gently popped off/open using a small screwdriver. You will most likely need to grab the entire assembly from below with fingers, a screwdriver, or pliers to keep it from rotating as you turn the bolt. Once it breaks free the nut will likely be incredibly easy to hold as you unscrew the bolt.