What are Tropes?
Tropes are cliches, common themes, or trends in books.
Take the murderous butler, for example. So many detective thrillers have used ‘the butler was the killer all along’ plot twist, it has become a comedic gag. There’s also the ‘she woke up and realized it was all a dream’ cliche. As far as I know, Alice in Wonderland was the first book to do this. In my opinion, this trope is lazy storytelling; the author couldn’t think of a good ending, so he says it was all a dream. Yet this trope caught on to other stories, unfortunately.
Many writers are advised against using tropes because they are overdone and predictable. And that is correct. Do not blame the butler or wake up the protagonist at the end of the book. The reader will be disappointed in you. And I will be in disappointed you. And you don’t want that.