It Follows is a modern horror movie classic written and directed by David Robert Mitchell in 2014. Despite its low budget, it currently has a Rotten Tomatoes score of 95 percent. If searched on YouTube, It Follows will show thousands of search results of reviews, analysis, and videos of people trying to figure out what the message behind the movie was.
If looked into deeply, this movie is more than it appears to be. On the surface level, the plot is about a demon virus of sorts that attaches itself to the last person an 'infected' person slept with. 'It' follows that person, walking slowly, until it catches up to them and kills them violently. In order to not be killed, the characters have to pass on the 'disease' through sexual intercourse or be prepared to outrun it the rest of their lives for as long as they can. If it catches the last infected person, it will go back down the chain of infected people, killing them. It is implied that 'it' will continue down the line of people until they are all dead. Only infected people can see the demon, but it appears to be physical, as it can interact with people even if they aren't infected.
The first theory of what 'it' actually is, would be a metaphorical representation of growing up and getting old. The main character, Jay, has graduated high school and is going to college. She chose to live with her mom and attend her hometown university. This shows Jay is still trying to hold onto her youth a little longer. The first scenes are of Jay in a pool, suggesting she is in the summer of her life. Jay frequently talks about how she used to be excited about growing up, getting older, and having the freedom that comes with being an adult. Now that she is older and in the fall of her life, she doesn't know where to go. She feels lonely and stuck in her life. And when she has sex for the first time, she feels she has lost her innocence and her youth. She fears the winter to come.
'It' walking slowly is a symbol of how time marches forward slowly. It will catch up to everyone, and that is the fear Jay feels. Jay sees the representation of time as an elderly woman, a woman unable to dress herself and can't control her bodily functions, and as her dad who is presumed dead. Whenever Jay begins to see old age coming for her, she runs away to a place she is comfortable and reminds her of her youth. Examples of this being running to a swing set, getting ice cream, and going on a road trip.
Another theory is that 'it' is a representation of the past, seen by all of the old technology within the film. When a person can't let go of the past, it can haunt them forever. In Jay's case, the past is her dad and her ex-boyfriend, Hugh. The old saying "hurt people hurt people" can also be applied to this theory. Hugh wasn't able to let go of his past and was hurt by someone, so now he passes on that hurt to others. It's a vicious cycle. 'It' could also be seen as a physical embodiment of karma, and it will eventually get back to the person who started the mess. 'It' is also able to interact with the characters in the film that aren't infected, suggesting that even though Jay was the one who was hurt, that infectious sorrow can hurt the people around the one who has been done wrong.
The last theory is that 'it' is a representation of sexual assault. This is the most obvious theory but may have a deeper backstory. It's possible that whatever it is now may have been human at one point. If that is the case, then this can be seen as a revenge story of someone who was assaulted and is now coming for the one who hurt them. But as mentioned before, hurt people hurt other people. So it's possible this demon is trying to find the person who first began it all, if they're even still alive.
The movie shows how an unhealthy view of sex can start early, even as a child. For example, the children who watch Jay while she swims or when she is dressing are showing early signs of this unhealthy sexual fascination. Jay and her friend Paul also have a conversation about finding adult magazines as children and trying not to get caught with them. The line "it could look like someone you know or a stranger in the crowd" suggests that a sexual assaulter could be anyone.
When the movie ends, it is implied that Jay and Paul are still being stalked. In order to survive, they cling to each other. Everybody needs someone every now and then to help them through difficult times or to help them transition into a new chapter in life. Whatever 'it' is, it's always good to have a friend to go through it with.