Light, it was already known, moved at a constant speed.
If light was moving at a constant speed, it begged the question – constant relative to what?
That something was proposed to be the all-pervasive ETHER.
Someone moving against ether would measure slower speed; with ether, higher speed. This was analogous to how sound waves were perceived as humans moved towards or away from the source.
That someone could also be considered to be the planet Earth. As Earth moves through same ether, riding along IN THE DIRECTION of earth’s revolution, one should measure a higher light speed; whereas going perpendicular to the direction of motion, should give a different speed.
Yet, as the famous M and M experiment proved, both speed were found to be exactly the same, no matter what direction they were measured in.
Einstein proposed that the whole idea of the ether was flawed; light did not need a medium to be measured against – the medium, if any, was spacetime itself. In order for all moving observers to measure the SAME SPEED, regardless of their motion, something else must have to give. That something else was time! Everyone, in essence, had their own clock and their own verison of what the current time is!