After reading some,
I believe the theory is, that when say you travel with in a car that's going 50 mph, another car passes also going 50 mph, it seems that the car is going 100mph. BUT when going almost the speed of light this is not the case...
Say you travel in your spaceship with 0.99 the speed of light and another spaceship passes also going 0.99, it still looks like it's going 0.99
Then what was what I said before?