Back in the 1700s scientists were still struggling to understand which theory of light was correct: was it composed of particles or was it made of waves? Under the theory that light is waves, it was not clear how it would respond to gravity. But if light was composed of particles, it would be expected that they woud be affected by gravity in the same way apples and planets are. This expectation grew when it was discovered that light did not travel infinitely fast, but with a finite measurable velocity.
In 1915 Einstein proposed The Theory Of General Relativity. General relativity explained in a consistent way, how gravity affects light, and also as a direct consequence - time. Einstein presented the idea that gravity was merely a curvature of space-time created by mass. One popular model of explaining this is to picture a smooth sheet or blanket stretched out above the floor. Now place a bowling ball on it, and it's weight (mass) will make a dent in the fabric (space-time). Anything else you put on that sheet will roll toward the ball, and in effect, the ball's mass has created a gravitational pull. As per Einstein's theory, the gravity bends the space-time curve such that any mass, and also light, will follow the bend in our fabric.
In this new chaotic Universe, Einstein's calculations needed a measuring stick which was stable,unchanging, constant and invariable, and one which would remain as a reliable benchmark for any observer regardless of their position in space. It would have to be viable for measuring the incredibly long distances between far away galaxies, but then could also be applied just as accurately to the speed travelled by atoms. His moment of genius came in realising that from any given frame of reference all light travels at the speed of light. Einstein's theory was not about how everything is relative to the observer, but rather about what it is that is independent of them. New thinking about the speed of light is starting to reveal it's something which is not independent of an observer, but rather, the speed of light is actually dependent upon the observer.
We often define distances by the time it takes light to travel between two points. For example, one light year is the distance light will travel in a year. Our notion of time is therefore is a product of how long it takes an entity to travel a specific distance. For example it takes 365 days for the Earth to orbit the Sun, and this is how we form the measurement of one year. Our notion of time is therefore a product of how we observe and measure velocity, but it fails to account for the rate of perception with which all observations are made.The mix-up arises from thinking the speed at which the brain observes the speed of light is constant, but in theory, the brain could be modified to think even faster, so that in effect, reality would play in slow motion.