300 years passed on Earth, on the spaceship only 80 years have passed. It gets funky so you just need to read from bunch of sources till it click.

For example, everything moves at C on the spacetime continuum. Since we are moving very slowly through space, it means most of our speed happens on the time part of the spacetime. Since light moves at speed of C, from the POV of the photon, it has no travel time, it just teleports from where it is emitted to where it hits something.

In other world, if we surpass all limits and build a ship that moves at 1C. Whoever is piloting it would never know when to brake because there would be no time flowing.

Edit: break > brake