Rice Krispie Treats are a confectionary made by combining Rice Krispies cereal into marshmallow and butter. They are highly popular in the United States and most Americans have eaten one.
Steve when he went on a diet along with doing a failed single jump by jump rope and half a sit-up in Season 3's "High Stakes" mourned that he wasn't losing weight (mainly cause he was gaining it fast) and Steve subsisted on eating just these "rice cakes" only for Denzel to angrily lecture Steve's eating something else.
Rice Krispie Treats are high in calories, sugar and fat, so they're probably not the best food to frequently eat when you're on a diet. Do you wanna get fat like Steve?