In the introduction, the author explains that many people have false impressions of what a black hole really is. I think she wants people to know the truth about why black holes are an important part of the universe.
The author wants to correct a false impression and to clear the air. Black holes are an important part of the universe, but unfortunately many of the people do not understand what a black hole really is and how important it is.
It is a compound sentence - a sentence that is composed of two simple (independent) sentences connected together -- in this case by the coordinating conjunction 'but.'
