It Follows, narratively, isn't anything to write home about. It's your average young adults sleeping around without caring about the consequences; although in the case of the film, the consequence can be death if you sleep with someone that is carrying the weight of a curse that comes for whoever currently possesses it.
Pass the problem onto the next person, and hope they continue to do the same. If not, the curse rolls backward down the line and eventually lands on you again, and there isn't really any way of knowing until it's almost too late.
Outside of the narrative is where It Follows works so excellently at being a horror and thriller: the film uses various methods of displaying whether the curse is slowly approaching in certain shots. For example, the protagonist is sitting in a car with her friends; in the background, far into the horizon can be found the curse, slowly walking and approaching the car. None of the characters are aware, and that's what makes it so good. It induces this constant paranoia into the viewer, more-so than it does in the characters. The viewer is then constantly searching in the background, trying to find out whether it is following or not, and usually it is.
As stated, its narrative is average, so don't expect an excellent story to accompany those eerie moments and excellent cinematography when it appears. That said, those moments certainly justify everything else the film lacks in