Learn Swedish: Det diskas och städas bakom kulisserna. - Washing up and cleaning are being done behind the scenes.

This is an impersonal construction using a presentational det. By combining the empty subject det (it) with passive verbs, it translates to "washing up is being done" or "there is washing up happening". It puts all the focus on the action itself, completely removing the need to mention who is doing it.

Why do diskas and städas end in -s?

This is the s-passive form of the verbs diska (to wash dishes) and städa (to clean). Adding the -s changes the active "cleans" to the passive "is cleaned". When paired with det, it is the standard way to describe a general, ongoing background activity.

Could I use man (one) instead of this passive structure?

Yes, you could say man diskar och städar (one washes up and cleans) or even någon diskar (someone washes up). However, the det + s-passive structure is very common in written and spoken Swedish for setting a scene where the people doing the work are completely irrelevant to the story.

What is the literal meaning of kulisserna?

A kuliss is a theatrical term for a stage backdrop, wing, or piece of scenery. Just like in English, the definite plural phrase bakom kulisserna is used both literally in a theater and figuratively to mean "behind the scenes".