It is the fixed rate at which all mass-energy is moving through space-time. The rate never changes for anything. If an object is moving slowly through the space

It is the fixed rate at which all mass-energy is moving through space-time. The rate never changes for anything. If an object is moving slowly through the space axis of space-time then it is moving quite rapidly, though at a fixed and predictable rate, through the time axis, and vice versa. C is the rate at which time and space are flowing in the simulation. Clock speed is a good analogy.