Who says you can't travel 100 light years in 1 second? Isn't 100 light years the distance light travels in one year in the Earth's frame of reference? So, if we have that distance, then something traveling very close to the speed of light (I attempted to calculate how close, but had rounding errors on my calculator) could travel that distance in 1 second, relative to them, which would be 100 years, relative to the earth.
Anyway, perhaps this explanation might help. (although it's not perfect, scientifically)
The speed of light is exactly 299,792,458 meters per second (by definition)
Now, for a moment, imagine you're in the concorde airliner, capable of supersonic flight (faster than the speed of sound). While the jet is traveling faster than the speed of sound, the person 2 rows behind you yells "omg! There's a fire in the engine!" Does the sound reach you and cause you to panic with the rest of the passengers? Yes. That's because the medium through which the sound is travelling from passenger B to you is stationary with respect to you and that passenger (this is a horrible analogy at this point, but it might work.)
Now, since the concorde has been banned, and after it lands safely with that bad engine, aliens come to earth and replace the engines with something making the jet capable of flying at nearly the speed of light (with respect to the earth) The speed of light, again, is 299,792,458 m/s. The jet is capable of traveling at 299,792,457.9 m/s. Again, the guy two rows behind you starts yelling, but he says something more like "holy ******, did you see that alien pass us? That guy is a maniac." Again, it should seem plausible to you that you're going to hear him almost immediately. And, you see his mouth moving while you hear him talking. Then, he pulls out a green pen laser from his pocket, shines it toward you, where it just misses your head, goes into the cockpit and out through the windshield.
If you've followed along so far, you need to stop and think about this: when that laser passes through windshield, it's actually going to be going slightly faster (light travels faster in the vacuum of space than it does in air.) Suddenly, the spaceship crashes into an interplanetary chicken. Frozen of course due to the extremely low temperatures, and it goes through the windshield, releasing all of the air from inside the space ship. Now, the passenger 2 rows behind you blinks out "S O S" in morse code during his dying seconds. Of course, the light is now traveling at c inside your spaceship, no longer going *slightly* slower due to the index of refraction of air.
Did I mention that the aliens put giant headlights on the jet? Now, as those flashes of green light go forward, relative you to, they are traveling forward at the speed of light. Just about this time the space ship is going by the earth.
Does an observer on earth see the light traveling forward at nearly twice the speed of light? Can't happen.
But, how can an observer on earth see the light coming from your ship and moving at the speed of light, if your ship is moving at the speed of light? There's only one way possible: the rate of change of time is different for you on the ship than it is for us on earth.
Einstein's idea was that the speed of light was a constant, rather than the speed of time.