That is the rosinbag. Rosin is a substance (i don't know what), but it's white and powdery and it helps with grip. You see gymnasts rubbing it on their hands right before they do the paralell bars.
to legally play only one person can be on the pitchers mound while the game is in play
The diameter of an MLB pitcher's mound is 18 feet.
Measure from the front edge of the pitching rubber to the back tip of home plate.
More than likely a pitcher's mound is removed by smoothing the dirt out with a bulldozer.
The mound allows pitchers a much better chance of pitching within the strike zone.