This is the 287th most frequent Polish word.
muszą
"Muszą" means "they must" or "they have to" in English.
Here, 'muszą' is used to express necessity or obligation in the context of plural third-person subjects.
Oni muszą iść do szkoły.
They must go to school.
In this sentence, 'muszą' is illustrating a requirement that must be fulfilled by the third-person plural subject.
Nowi pracownicy muszą poznać procedury.
New employees must learn the procedures.
'Muszą' demonstrates a rule or expectation directed at the third-person plural subject group.