There are three dimensions of space and one dimension of time. This space time is kind of like a big bed sheet made out of jello. You can warp it to your command and space time can even be broken (Black holes are the holes that rip through space time.) This space time bends depending on how fast a celestial object is traveling.
Albert Einstein proved in his theory of special relativity that you can travel through time if you move faster than the speed of light. It also says the faster you travel, the more mass you obtain. That's why you can't go faster than the speed of light: your mass would become infinite when you get to 99.99 percent of the speed of light. You would then need an infinite amount of energy to boost you beyond the barrier. If you went faster than the speed of light, you would travel through time. If you could travel at 80% the speed of light. if you left Earth for several months, several years or even centuries could have passed on Earth.
The second way to travel through time is to go through a wormhole. Wormholes can shorten the distance between two points in space. Instead of taking 3 years to get to a certain location, it only takes 3 weeks to get to the same location. The problem with wormholes is that you need a HUGE amount of energy to open a wormhole and to sustain it. The process would destroy the entire Earth. Also, the wormhole is only a one way trip. It would take you decades to get back to Earth.
People also wonder what would happen if you went back in time and you changed something. This concept was used in the 1985 movie Back to The Future when Marty McFly travels back to 1955 and causes his mother to fall in love in him instead of his father. If he screws up, he won't exist anymore. There are tons of other anecdotes of the results of time travel in science fiction, many of them too long and detailed for me to tell. That's all for now. I hope you enjoyed it.