It is speed, the speed of light. Particles that have zero mass go at the speed of light. This is a property of the particle. That there is a maximum speed is a

It is speed, the speed of light. Particles that have zero mass go at the speed of light. This is a property of the particle. That there is a maximum speed is a property of spacetime however. Even if there were no massless particles we could still discover special relativity, for example by noticing that when you speed up particles they can't get past a particular speed. In the large hadron collider they can speed up particles to 99.9999991% of the speed of light.