Since Dwarves consider "low" to be near the top and much more desirable than "high", their supreme leader is known as the Low King. He is actually elected by a council; "king" in Dwarvish (dezka-knik) really means the supervisor or chief engineer of a mine and the Low King supervises them. The Low Throne is both his literal and figurative seat. The physical throne should contain the Scone of Stone if someone hasn't stolen it.
The Low Throne is a parody of many a 'High Throne' such as that at Cair Paravel in the Narnia series by CS Lewis.