Genres come with reader expectations, and expectations come with certain stereotypes connected to genres.
A genre is a style of category of art, music or literature.
An expectation is a strong belief that something will happen or be the case.
A stereotype is a widely held but fixed and oversimplified image or idea of a particular type of person or thing.
For example; romance is a genre, the expectations are a sympathetic heroine, a strong and irresistible hero, emotional tension and a happy ending (it would help if the plot was believable) and the stereotypes are the bad boy and the good, shy ‘nerdy’ girl or the damsel in distress and the prince that saves her. I.e. Pride and Prejudice, Me Before You, Cinderella, and Beauty and the Beast.
For a genre to work the reader will have expectations about the story, and these expectations come from the stereotypes that have been placed on the genre. Authors need to be aware of what expectations readers will have if they’ve written a novel in a particular genre, and use that awareness to either meet these expectations or subvert them in a way that readers will find appealing and interesting.