We are aware of physics, right. Well, physics is the scientific study of the matter and its state along with the behavior of the object in space-time. Modern-day scientists broadly use two theories to explain physics. They are: Theory of relativity (for massive objects) and quantum mechanics (for atomic and sub-atomic particles). Here, I'm going give a simple explanation about special theory of relativity given by Albert Einstein. Well, Albert Einstein was very curious since his childhood. He wanted to know what would it feel traveling on the light. His eagerness led the foundation of the theory of relativity. Theory of relativity has been divided into two parts :
1. Special Theory of Relativity (1905) and
2. General Theory of Relativity (1915)
Let's talk about Special Relativity now. As per Michelson-Morley experiment, we have known that the speed of light is constant i.e. 299,792,458 m/s. But this created a problem by solving which Albert Einstein discovered special relativity. First, let's take an example to sort out the problem. Suppose, 'A' is standing on the top of a train moving at a speed of 50 m/s whereas 'B' is an observer. For 'B', 'A' is moving at the speed of 50 m/s in the forward direction while in perspective of 'A', 'B' is moving in a backward direction at speed of 50 m/s. If 'A' throws a ball at the speed of 20 m/s in train's direction, then for him the speed of the ball will be 20 m/s but for 'A' it will be (speed of ball + speed of train)= 70 m/s. Similarly, if 'A' turns a flashlight then for him the speed of light would be 299,792,438 m/s whereas for 'B' it will be (speed of light + speed of the car). But this will make the speed of light variable and create a contradiction on Michelson-Morley experiment.
Albert Einstein tackled this problem and declared that the speed of light is constant and no object can travel at a speed greater than the speed of light.
As we know, Speed = (distance traveled) / (time taken)
Therefore, to keep the speed of light constant distance will be shortened ( length contraction) and time will become slow (time dilation). Therefore, if an object gets closer to the speed of light its time become slower and its length will be contracted in comparison to its length at rest.
Besides this, Einstein also gave the famous energy-mass equivalence equation i.e. E= MC^2 where
E = Energy , M = Mass and C= Speed of light. Using this equation, we can convert mass into energy and vice-versa.