Meaning of “must” - Learner’s Dictionary

must

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