Because one student had bad food allergies, the school put a restriction on what kinds of food the other students were allowed to bring for lunch.
A restriction is a rule or limit that tells you what you cannot do.
On his first day at work, Kwame's new boss said: "Let me tell you about our restrictions. No wearingjeans. No taking breaks. No sitting down at work. Oh, and you are not allowed to take any days off for the next six months. Welcome."