Learn Hardcore Norwegian: Det er for mange folk på bussen. - There are too many people on the bus.

It’s the existential construction equivalent to There is / There are in English. Det is a dummy subject (it doesn’t refer to anything), and er is the present tense of å være (to be).

Why do we need det at the beginning? Can’t we just say Er for mange folk på bussen?

In statements, Norwegian normally requires a dummy subject. Without det, the verb has no subject, so Det er is the standard pattern for "there is/are" sentences. You can drop det only when you invert to form a question: Er det for mange folk på bussen?

What does for mange mean here?

For functions as too, and mange means many. Together, for mange means too many.

Could I say for mye folk instead of for mange folk?

Generally no. Mye modifies uncountable (mass) nouns, while mange modifies countable ones. Folk (people) is treated as countable in this context, so for mange folk is correct. For mye folk would sound odd or very colloquial.