If you had a train that went round the earth at 99.999% of the speed of light....
...that is, around the earth 7 times a second...
...and you put a person on that train, and they decided to run from one side of the carriage to the other...
...then they would be running faster than the speed of light, right?
To stop this happening, time slows down inside the train. The same way that time slows down when the Large Hydron Collider speeds up particles really fast and they survive 30x longer than normal.
In the train example, if the people in the train stayed in the train for 7 days, 100 years would pass outside the train. How mind boggling is that?
Similar thing happens with gravity. Big heavy things dilate time. So like in the center of our galaxy is a massive big old black hole. And time around that black hole passes 2x slower. So if you had twins and one stayed on earth and one went in a spaceship around the black hole, one of them would age twice as fast! Then you could look at the old you and see what you'll turn out like.
To get a little more sci-fi, wormholes. We dont know much about them, and its all theory. But, in theory, potentially maybe backwards time travel could be possible. But then you get loadssss of paradoxes like the grandfather paradox