By setup here I mean character details, their backstory, the main challenge of the plot, etc. When you read the book, you would come to know this hidden key detail (at least) by 1/3rd of the book. These are the details that makes you care for the characters and make you want to read the book. It is not the detail that you wait whole book to know (how will the hero destroy the one ring?)

For eg:

“but their quiet life is thrown into turmoil when an uninvited stranger shows up at their door with a startling proposal.”

what is the damn proposal? I am not reading 100 pages of story to come to a “proposal” if it does not sound interesting to me.

Now. this is not a complaint about the book (it is an interesting book here, and the proposal is that the man has to leave his wife he dearly loves, to go and stay in a space station for 2 years.)

This kind of synopsis feels like a cheap trick to me, to get me to read a book that may not be worth my time, while paradoxically the book actually might be good.