We have two conflicting rules:
1) Nothing can move faster than the speed of light; and
2) For every action, there is an equal and opposite reaction (which mathematically is expressed as Force = Mass X Acceleration)
If you are traveling in a spaceship going just under the speed of light, say 0.9999999999999% this doesn’t break rule 1 or 2. No problem. But then say you throw a sofa out of the back of your spaceship. Rule 2 says you should speed up. But rule 1 says you can’t.
So the universe cheats.
Force = Mass X Acceleration has, as its basic units, Force, Mass, Distance, Time. To preserve Rule 1, the universe starts to mess with Mass, and Time. It slows time down in fact, and the faster you are going, the slower time goes.
In fact, when you really are going at 0.9999999999999% the speed of light, your rocket engine is really a time machine. You turn on your rocket and instead of going faster, you make time slow down more and more and more from your perspective.
You, personally, could travel across the galaxy in a few weeks if you just had a conventional rocket big enough. On earth thousands of years would pass, but for you, it would be virtually no time at all.