Special Relativity- A 60-Seconds Overview

To understand Einstein's special relativity, visualize two cars approaching each other at constant speed (acc.=0 m/s^2).

For a person sitting in the red car, the blue car is moving towards him whereas for a person sitting in the blue car, the red car is approaching her. Now, zoom out and imagine being a third person watching this motion. For you, both the cars are moving towards each other. In other words, their apparent speeds differ as per the viewpoint. (Like how we see trees sprint by from inside the train, while for the snow owl sitting on the tree, it is us who are rushing forward with the train)

The properties that changed when you changed positions of reference are the relative properties (i.e the apparent motion of the cars). Those that remain unchanged are the absolute properties.

The theory of Special Relativity states that the laws of physics are the same in all inertial frames of reference (i.e with zero acceleration)  and that the speed of light in a vacuum is always constant. Thus, they are absolute properties while entities like the speed of sound, the speed of a car, and the speed of a dog depend on the viewer's frame of reference.