Meaning of “must” - Learner’s Dictionary

must

modal verb us uk strong /mʌst/ weak /məst, məs/
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)