I think that means "DO NOT EAT SNACKS"or stuff between meals. because you will be full already without having meals on time.
Son: Mom, I'm hungry.
Mom: Can you wait for half an hour? Because dinner is almost ready.
In this situation, If the son doesn't wait and eats something before dinner because he is hungry, then HE will probably be FULL.
So if you keep eating something between meals, you will be full and have a bad habit in eating properly.
hehe Good Luck.