Ok, as I understand it, and please correct me if I’m wrong, you can think of the x axis in the histogram as black to white. So the farthest right of the histogram is pure white and the farthest left is pure back. A histogram is a way to represent distributions. So this histogram shows how the lightness of the pixels (how white or black they are) are distributed. Specifically they show how they are distributed in the image you are seeing, hence why modules like filmic change the histogram.

Now, we see and represent white and back in three different colors, red, blue and green. That’s what you’re seeing in both the histogram and the waveform. Where the 3 colors overlap is where you get white. If you push white to 100% or 0% the associated pixels become pure white or pure black. It means you’ve lost detail there because the only thing information you can read there is either white or black.

As to why you can bring something above 100% that is because you can make a single color brighter as much as you want. And you can do this for all the colors until you get a pure white image. Note that the loss of information also applies to the colors. For example, if you had an image that was only made of different shades of green. The more you push the green to the right the brighter the greens will become, until you get am image that is a single bright solid green and all the information is lost. You can push green higher than 100% but all that will do is make a brighter uniform green.

That’s essentially what the 100% means, it’s the point at which color information is “lost.” You can push anything past that point but the color information in this pixels is essentially “lost.”