Learn Dutch: Boven staan de stoelen. - Upstairs are the chairs.

Because the sentence starts with the location boven (upstairs). In Dutch, the verb must always be the second idea in a main clause. When you put a word like boven at the front for emphasis, the subject has to hop to the other side of the verb to keep the verb in second place. This swap is called inversion.

Is it wrong to say De stoelen staan boven?

Not at all! De stoelen staan boven is the standard word order (Subject - Verb - Location). Starting the sentence with Boven is just a way to put extra emphasis on where the chairs are. Just remember that if you move the location to the front, you must apply the inversion rule.

Why does the sentence use staan (to stand) instead of a word for "are"?

Dutch strongly prefers to describe exactly how an object is positioned in a space, rather than just saying it "is" there. Because chairs have legs and rest upright on them, Dutch speakers say they "stand" (staan) upstairs. Using a form of zijn (to be) is understood, but staan sounds much more natural.

Does boven always mean "upstairs"?

It generally means "above" or "on top". However, when you are talking about the layout of a house or a building, saying something is located "above" naturally translates to "upstairs" in English.