Every once in awhile I read a post where I understand nothing.
Think of it as being just like an x-ray of your body. X-rays shine right through you, for the most part, but the denser parts of you stop more of the x-rays. That's why images of your bones show up on the film: they're dense enough to stop most of the x-rays, while your bulging biceps and rippling pectoral muscles are not.
This method doesn't work on a pyramid, because it's too dense to let any of the x-rays through. Muons, however, can do the job, because they can travel through a whole lot of matter before they stop. The denser the matter, though, the more of the muons it will stop. If there's a hollow chamber hidden away somewhere in the structure, more muons will pass through that part of the structure than if it were just solid stone.
Furthermore, muons are constantly raining down on us from the sky, so you don't need to set up a beam like you do with an x-ray machine. If you just wait long enough, you can collect enough muons to make as detailed an exposure as you like.