Learn Portuguese (Portugal): Relaxa-se muito aqui, desde que haja silêncio na sauna. - One relaxes a lot here, provided that there is silence in the sauna.

It is triggered by the conjunction desde que. This phrase sets up a condition (meaning "provided that" or "as long as") for the first part of the sentence to happen. Because it is a condition rather than a guaranteed fact, Portuguese requires the subjunctive mood here.

Doesn't desde usually mean "since"? How does it mean "provided that" here?

Yes, on its own, desde usually means "since" for time or place. However, the fixed phrase desde que acts as a conditional conjunction meaning "provided that" or "as long as". (Note: desde que can sometimes mean "since the moment that", but when it sets up a condition like in this sentence, it always takes the subjunctive).

What verb does haja come from, and why is it used here?

Haja is the present subjunctive form of the irregular verb haver. It is used here in its impersonal sense to mean "there is" (the subjunctive equivalent of há). We use haver because we are talking about the existence of silence, rather than describing the state of a specific object.

Why does the sentence start with relaxa-se? Who is relaxing?

This is an example of the impersonal se. Instead of saying a specific person relaxes, it means "people in general relax" or "one relaxes". It is a very common way in Portuguese to make general statements where the specific subject does not matter.