It's of two men playing with a pool full of a non-newtonian fluid. Specifically, this is a dilatant fluid, meaning that its viscosity increases with shear stress. Since this probably doesn't tell you a lot, it means that the fluid essentially becomes more rigid as more stress is applied parallel to the surface of said liquid.
What the hell does all that mean? It means that these guys can run, hop, and skip across this liquid with minimal difficulty, but will sink into it and float if they stand still. It's cool as hell.
For those of you who are equally thrilled with all this, the stuff they're running across is commonly referred to as Oobleck and can be made in any reasonably well-stocked kitchen. You can use this recipe or just mix 1 part water to 1.5-2 parts cornstarch. Food coloring is optional, but kinda cool. A great, inexpensive science activity for the kids.