TWIN PARADOX is a paradoxical situation coming in while explaining the theory of relativity. “Time dilation” is one of the important consequences of relativity. It says that if a body is moving with relativistic speeds, time will dialate for it. That is, a duration of 1 second for him will be more than that for a person at rest or moving at ordinary speeds.
The relativistic time is given by the equation,
So, if we imagine twin brothers, one at rest at earth and other going on a relativistic interstellar trip (certainly he has to go out of solar system as the velocity of light is very much more than escape velocity of solar system, and further, he will reach the sun in just 8 minutes if traveled with the speed of light)
Let us take that his speed is 0.9 times the speed of light in vacuum. Then 1 second for him is equal to 27 s (approximately) for his brother on earth. So, if he returns after one year of space programme, he will be older by 1 year, but his brother who was at home will be older by 27 years or more; if alive.
This is the TWIN Paradox.
You are welcome to think of the possibilities of a relativistic travel and the related issues here.