So you can get some sand, treat it appropriately, and when it's poured into water the two simply won't mix. The water can't even get into the tiny spaces between the sand grains, so the air that's mixed in with the sand stays there as a big bubble, giving it that shiny appearance.
Now, just how hydrophobicity works is an interesting subject in and of itself. It's important not just in waterproofing, but how the proteins that your body uses can arrange themselves, and how certain drugs work. I'll be writing about that in the near future.