What is the significance of the relative speed of light in the theory of special relativity?

The significance of the relative speed of light in the theory of special relativity is that it serves as a universal constant, meaning that the speed of light is the same for all observers regardless of their relative motion. This principle leads to the concept of time dilation and length contraction, which are fundamental aspects of special relativity that explain how time and space are perceived differently by observers in motion relative to each other.