There is a ton of information out there that explains this, but in the interest of saving time, here it is.
His mother used to give him some skittles (power pellets) to keep his sugar level up when playing football, so it became a habit at a young age that he continues.
Skittles first gave him a year supply. Now it appears they have entered into an actual arrangement with Marshawn, which has already more than likely made them money since you couldn't find Skittles on store shelves two weeks ago.