This is the expanding balloon analogy of the universe. It's an oldie but goodie.

Imagine you have an expanding balloon. The 3D space of the physical universe is represented by the 2D surface of the spherical balloon. Ants live on this balloon, and they can run really, really fast. Say 186,000 mph is maximum ant speed. As you blow air into the balloon, the skin of the balloon stretches. Even though the ants are running at the same speed of 186,000 mph, as the balloon expands, they are able to run around a smaller fraction of the circumference of the balloon per unit time. At one point in time, they may be able to run all the way around the balloon in an hour, but after a few seconds of expansion, they're only able to make it half way around in an hour, then a quarter of the way around. The "measured velocity" of the ants is still 186,000 mph. If you were standing next to an ant as it raced by, that's the speed you would measure. However, if you're standing a distance away from the ant and watching it run toward you, it will seem to be running slower due to the expansion of the balloon. If the ant were standing still, you'd actually observe it to be receding from your position. If the skin of the balloon is expanding faster than 186,000 mph, then even an ant running at full speed would be seen to move away from you -- that is to say, the "coordinate velocity" of the ant would be negative. Now replace the balloon with the physical universe and ants with photons. This is why we observe a "horizon distance" in the universe. At about 1.3 * 10^9 light years away from us, the universe is expanding so quickly that even the light emitted from those distance sources cannot reach us.