Introduction: ALBERT EINSTEIN

Albert Einstein was a German-born theoretical physicist. He developed the general theory of relativity, one of the two pillars of modern physics. Einstein is best known in popular culture for his mass-energy equivalence formula  E = mc² .He received the 1921 Nobel Prize in Physics.

Einstein was Jewish so while he was visiting the United States, Hitler came to power in 1933 so because of that he didn't go back to Germany, where he had been a professor at the Berlin Academy of Sciences. He settled in the  U.S., becoming an American citizen.

Special theory of relativity

In the special theory of relativity, Einstein, in his first postulated, redeveloped the simultaneity saying that the speed of light is constant and independent of any sistem of reference. Also he created another postulated in which, he said that if a flash of light is thrown seen it by two observers, in relative motion, both see the light away producing a perfect circle with each of them in the center. If in both sides, we put a detector, none of the observers would agree on what detector was activated first. 
Because of that, we can confirm that now, with Einstein, the concepts of absolute time and simultaneity are lost. 

Time dilation

In the theory of relativity, time dilation is a difference of elapsed time between two events as measured by observers either moving relative to each other.

An accurate clock at rest with respect to one observer may be measured to tick a different rate when compared to a second observer's own equally accurate clock. This effect isnt a technical aspect of the clocks or it isnt from the fact that signals need time to propagate, but from the nature of spacetime itself.

The relativity of simultaneity

The relativity of simultaneity isn't absolute, but depends on the observer's reference frame. 

Anyway, it is easiest to explain with an example:

Two observers, Tom and William. 

Tom is on a train car. The train goes too faster but always in constant speed. Tom is in the center of the train car and he throws a ray of light. That ray of light arrives at the walls in the same time. For Tom, they are simultaneous events. It is in the following image. 

William is on the train station. William sees two movements, the ray of light and the train. (I remember you that the speed of light is constant in all references systems).

William observe that the ray of light traveling in the inverse direction of the movement, reachs the wall before the ray of light which is traveling in the direction of the movement. We can see it better in the following image. 

In conclusion, we can analyze that all we see depends of the position of the observer. There is a different reality for each observer and the simultaneity is relative.

Twins paradox

In physics, the twin paradox is a thought experiment in special relativity involving identical twins, one of whom makes a journey into space in a high-speed rocket and returns home to find that the win who remained on Earth has aged more. 
In his famous paper on special relativity in 1905, Albert Einstein deduced that when two clocks were brought together and synchronized, and then one was moved away and brought back, the clock which had undergone the traveling would be found to be lagging behind the clock which had stayed put. Einstein considered this to be a natural consequence of special relativity and time dilation.