You got these twin brothers, see...
One takes off on a space ship, going to point A.. and he then will return..
The round trip, at the speed of light, will take say, 10 years..
Space twin returns to earth in what, to him, was 10 years..
Space twin finds his earth twin has aged say, 50 years, and is now 40 years older than brother space twin..
As you approach the speed of light, time itself is changed.. or how we (travellers) experience time..
The "basic" explanation is that time slows down..
I'm sure that's not accurate, but for the non-physicist layman, it's close enough..
The idea is, when at say, 60 % of light speed, it takes you 200 years to take a sip of your coffee.. to you, it's only 5 seconds.. for anyone observing you from outside of your rapidly moving craft, it's 200 years..
That's where the controversy comes in concerning faster-than-light travel...
Some (sci-fi authors especially) have postulated that travelling FTL would or could result in "time travel", actually reversing time..
Whether time would reverse within the traveller's ship, causing them to get younger is a problem... but, at any rate, travelling to another solar system, or even another galaxy would then be "theoretically" possible..
Simply "time" your round trip FTL travel to bring you back within say, a year of your departure..
All sorts of rules concerning time paradox says this is extremely dangerous, if not impossible..
More than you ever wanted to know about relativity and the space time thing.
It be like flying from Atlanta to Birmingham in one of dem fast jets, dat thing is so fast, it get der before it take off.