Meaning

• Something that is mandatory or required.

  "If you'll be out all day, a map is a must."

Meaning

• (modal auxiliary, defective) To do with certainty; indicates that the speaker is certain that the subject will have executed the predicate.

  "If it has rained all day, it must be very wet outside."

• (modal auxiliary, defective) To do as a requirement; indicates that the sentence subject is required as an imperative or directive to execute the sentence predicate, with failure to do so resulting in a negative consequence.

  "This door handle must be rotated fully."

• (modal auxiliary, defective) said about something that is very likely, probable, or certain to be true

  "The children must be asleep by now."