Rice Krispie Treats are a type of confectionary made by mixing Rice Krispies cereal with marshmallow and butter then baking them. They are highly popular in the United States up to the point most Americans have eaten one before.
After getting too fat for his already oversized uniform, Steve went on a diet and after 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 (because 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 binge eat when you're on a diet. Do you wanna get fat like Steve?