Representation is simply how media portray certain types of people or communities, such as gender or racism.
The movie «Get Out» is a great example of how representation is used with race. The movie portrays a lot of the liberal kind of racism. It applies common horror tropes to reveal truths about how pernicious racism is in the real world. In the movie, the white society is completely conscious of the evil that is being done. The movie represents how, nowadays, a big part of the white society still makes someone from another race feel. Furthermore, it enables the viewers to empathize with the fears of black men.
This video explains the «deep horror of racism» in the movie get out.