Luminiferous aether
The easiest way to see the general idea is to imagine a very fast spaceship moving at one half the speed of light from one star to another. The spaceship has two light-speed meters, one pointing backward and one pointing forward. Since the spaceship is moving away from the photons given off by the yellow star, and running toward the photons given off by the blue star, our ordinary experience leads us to expect that we would measure the speed of the yellow photons as 150,000 kilometres per second, and the speed of the blue photons as 450,000 kilometres per second. However both photons are measured at 300,000 km/sec. Thereby this result indicates that speeds do not change relative to the movement of the spaceship.