Rice Krispie Treats are a confectionary made by mixing Rice Krispies cereal with marshmallow and butter then baking them. They are highly popular in the United States and most Americans have eaten one before.
After he got too fat for his already oversized uniform, Steve 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". He mourned that he wasn't losing weight (mainly cause he was gaining it instead) and Steve subsisted on eating just these "rice cakes" only for Denzel to angrily lecture Steve he's eating something completely different.
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?