A flat bed sheet that a lazy, couch potato-type person spreads out on his or her sofa to protect the upholstery from little piles of foods (especially from Fourth Meal) that he or she keeps at hand. Using a snack sheet saves the effort of washing dishes and bowls. Oftentimes, the person (or people) utilizing the snack sheet will do so unclothed. In this case, the snack sheet can further protect the couch upholstery from skin oils and other body soils. Also: a metonymy for sloth and gluttony
They never leave the house on a weekend. They just stay home, watching Lifetime and eating Ritz crackers on their snack sheet.