to have to; to be obliged to

You must stop at the lights

She said we must not park outside the house

to be required or necessary

Propaganda, to be effective, must be believed. To be believed, it must be credible. To be credible, it must be true — Hubert Humphrey

to be supposed or likely

It must be lunch time

used to express insistence

You must stay for dinner

used to express certainty, usu in ironic contexts

You must be joking