Spacetime is relative. Imagine this situation. You have a twin. Right after birth, one of you (probably you because you are more adventurous) boards a rocket ship to orbit the earth for forty years. When you return to earth, you will have aged less than your twin. This is an example of time dilation.
A real life example of this recently occurred when the astronaut Scott Kelly spent approximately one year aboard the International Space Station (ISS). Scott Kelly has an identical twin brother, astronaut Mark Kelly. When Scott Kelly returned to earth he had aged 13 milliseconds less than his brother.