First, Galilean relativity, that states that the physics describing how things move around works whether you yourself are moving or not. Still, there’s no problem with there being some particular “stage” with everything in the universe moving around on it. In Galilean relativity, the size of objects, the distance between two events that happen at the same time, and time itself are all fixed. Just like in Galilean relativity, different observers can disagree about where an event can happen, but unlike Galilean relativity, in Einstein’s relativity they can also disagree on when. If there’s nothing at all in the universe to base a reference frame on, then how do you define distance and time?