Einstein’s Theory of Special Relativity, first postulated in 1905, says that the laws of physics and the speed of light are the same for all observers, regardless of their own speed or motion. To have a better understanding, imagine two people travelling at different speeds observing the same beam of light. According to Special Relativity, both will record the same speed for the beam, regardless of their own speed and direction.
This contradicts more practical examples on Earth. If a car moves at 40mph away from an observer, and another travels at 50mph from the same point in the same direction, relative to each other the second car will be moving at 10mph. However, a light beam moving in the same direction as the cars would appear to have no change in speed relative to them and would remain at its universally agreed value c, about 299,792,458 metres per second. Of course, this is theoretical and in practice not measurable, but nevertheless it’s the basis of Einstein’s Theory of Special Relativity.