Essentially the closer you move at the speed of light, time flows slower for you. So, if you are in a rocket which is moving at close to c, for you the journey to Alpha Centauri which is 4 light years away may be like just an year. But for us outside of your rocket, it would likely be more than 4 years.

If you were to goto Alpha Centauri and then back to Earth, you may have spent just 2 years in your rocket. But when you get out of your rocket, you will find that 8 years have gone by on Earth.

Taking a lot of liberties with my explanation, but hope this ELI5 works.