This subgenre of horror involves human bodies… all kinds of disturbing imagery of them. Sometimes referred to as biological horror, body horror movies, tv, stories, and art emphasizes the alarming violations of human body parts and how distressing it can be.
Arguably Mary Shelley’s Frankenstein could be considered one of the earliest examples of body horror. An entire story or movie can be heavily entrenched in the subgenre, such as many David Cronenberg films. Or numerous depictions of Lovecraftian horror. Or specific parts or scenes can lend themselves to body horror while mostly interested in developing tropes in a different subgenre, such as Event Horizon (Sci-fi horror) or An American Werewolf in London (Lycanthrope horror).
Using the grotesquery of the human body in its different forms can be used as metaphor for many things. For more reading on the topic:
Mutations and Metamorphoses: Body Horror is Biological Horror
A Quick History of Body Horror in Cinema