Carry-on restrictions refer to the rules set by airlines regarding what passengers can bring on board in their small luggage. Typically, each airline allows one carry-on bag and one personal item, like a backpack or purse. The size and weight limits vary by airline, so it's important to check their specific guidelines before packing. Additionally, there are restrictions on certain items that cannot be taken in carry-on luggage. For example, sharp objects like knives and liquids over 3.4 ounces are generally prohibited. Understanding these rules helps ensure a smooth boarding process and keeps everyone safe during the flight.