Eraserhead, David Lynch's 1977 student film that has become a classic, is also Lynch's most direct example of a horror movie. All of his movies and other works have some element of the unsettling in it, but this is the only movie that would primarily be called "horror". Although, of course, even that is a simplification. Lacking dialog and a clear plot, it is more of 90 minutes of unsettling imagery and sets, rather than anything that could simply be called "Horror". There was a reason that we had to invent an entirely new English word to describe this movie.
The plot and setting of the movie are actually pretty simple to explain: Henry is a man living in what may be a stylized version of our contemporary world, or may be someplace, or sometime, else. He has a girlfriend, Mary, and while having dinner with Mary and her family, he learns that his wife has given premature birth to a deformed infant. Back in his small apartment, Mary attempts to care for the infant, but flees from its crying. Henry, caring for his sick infant, has a series of fantastic and frightening visions, leading to the film's climax.
Since it has been forty plus years since its release, and this film has been critically reviewed by many people, there is some general consensus to what it is "about", even though David Lynch is famously taciturn to confirm or deny deeper meanings to his films. It is generally agreed that the film's main theme is a fear of intimacy and parental responsibility, with an additional theme being urban decay and alienation. There are additional interpretations that could be made: while watching, I started sketching out a theory of the movie's events from a straight science-fiction point of view.
The most generic insight I can give into the horror genre is that horror provides a mediator for common anxieties. An only slightly less generic insight is that those anxieties are often topical to cultural shifts at the time. Pod people are communism and promiscuous teens being stalked by Jason are fallout from the sexual revolution and the Blair Witch is being trapped by our own media creations. Most horror movies have a fairly obvious relation to societal conflicts of their time, if not to generalized human anxiety. Eraserhead is no exception. Worms and pustules are frightening at any time. And in its themes of the intersection between sexuality as an act of aggression and sexuality as an act of procreation and nurturance, it is not a big leap to view it as a comment on society's (and the creator's) anxieties as family structures changed in the 1970s. The vast and oppressive urban landscapes can also be tied, easily, to the dehumanization of urban blight. All of these things are true. Despite being a surreal film, some of these themes are pretty obvious.
But while, and after, watching the film, even though I had that little toe hold into Eraserhead, does not mean I was not thinking of other things. I was. My imagination was all over the place. To say this film is about fears of sexual intimacy is true, but it is true in the same way as saying that Moby Dick is aboat boats is true. The most obvious and horrific part of the movie is the tip of the iceberg, and below that lies David Lynch's vast imagination --- and yours.