Meaning of “must” - Learner’s Dictionary

must

modal verb us uk strong /mʌst/ weak /məst, məs/
Extra Examples
To qualify for the competition, you must be over 18.Items must be returned within fourteen days of receipt.The delivery company must pay for any breakages.All cigarette advertising must carry a government health warning.Children under 10 years of age must be accompanied by an adult.
NECESSARY

A2 used to say that it is necessary that something happens or is done:

The meat must be cooked thoroughly.
You mustn't show this letter to anyone else.
I must get some sleep.
LIKELY

B2 used to show that you think something is very likely or certain to be true:

It must have been very upsetting for her.
You must be exhausted.
She must be very wealthy.
SUGGEST

B1 used to show that you think it is a good idea for someone to do something:

You must come and stay with us some time.

(Definition of “must modal verb” from the Cambridge Learner’s Dictionary © Cambridge University Press)