Let me see if I can try to explain this.

First off, light isn't just the fastest thing we know of, it is physically impossible to go faster than light according to the laws of physics as we understand it. This is because the speed of light is actually tied to the way spacetime works.

Imagine you are standing and you throw a ball. The ball travels at whatever speed you throw it, let's say 5 mph.

Now, let's put you on a train traveling at 20 mph and do the same thing. If you throw the same direction the train is traveling, your 5 mph adds to the train's 20 and the ball goes at 25 mph according to someone standing next to the track. Throw it the other way and they see it travel at 15 mph. To you, in either case, it appears to move at 5 mph.

Light doesn't do this. We've measured it, and in a vacuum light always appears to travel at the same speed (we call it c for short). If you hold a flashlight, your friend next to you can measure the speed of light and will find it to be c. If we put you back on that train and stand your friend next to the track, you will see the light moving at c, but so will your friend. Not c +/- 20 mph, but c. Even if we put you on a rocket traveling at some significant portion of light speed, say 0.5 c, both you and your friend would still observe the light from your flashlight to be traveling at c.

This is what Einstein figured out, and this is what we mean by Relativity. From this, we also know that objects moving faster experience an increase in mass (you have to get moving pretty close to c to really notice), and as you approach c that mass trends to infinity. That's why anything with mass cannot achieve the speed of light, it would be infinitely massive, and thus require infinite energy to accelerate to that speed. Thus, only things with no mass (such as light) can move that fast.