respected sir, please explain me briefly what is time dilation

Time dilation is the slowing of the passage of time experienced by objects in motion relative to an observer; measurable only by relativistic speeds.

 

If your mother goes around Alpha Centuri at a speed comparable to the speed of light when you are 18 and she is 38 when she comes back you are 40 and she is about 39. This is because it takes over 20 years to make the trip and time for her is far slower than it is for you on Earth.