The laws of physics are the same for any inertial reference frame, and the speed of light is independent of the emitting source and who receives it, being it (speed of light) constant in all inertial reference systems.
The postulates mentioned above, proposed by Albert Einstein, were the pillars for the development of the Special Theory of Relativity, which has as one of its implications the dilation of time.
In the mathematical expression mentioned above, we see that time is relative and depends on the reference frame in which it is measured.
Assume that it is feasible for a person to travel at 0.8c (with speed equivalent to 80% of the speed of light) with respect to an inertial frame of reference (x). At the end of her trip, she checks her watch (frame x’) and sees that it took 5 hours to complete it. For a clock that is in the x frame, it would have measured the time of this trip to be 8 h 19 min. (in approximate value).
From this example we can see that measuring the simultaneity of an event depends on the reference we adopt, and that time dilation is more noticeable in events in which the speeds are close to those of light, making this fact imperceptible in our daily lives.