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

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.