Must has 1 common meaning: Very strong necessity.
Necessity: something that must be done
• You must have a passport to travel.
*Notice: Must is used for very formal and strong warning or requirements. It is usually used on signs and documents. Parents, teachers and other people in authority also commonly use must to show that something is a requirement.
• You must come to class on time.
*Notice: Parents, teachers, and other people in authority will sometimes use “never” or “always” with must to heighten a warning. When someone uses “must never,” it is stronger than saying “must not.” When someone uses “must always,” it is stronger than saying “must.”
• You must not lie. Always tell the truth.
• You must never lie. You must always tell the truth.