Gender relations (the meaning)
Movie was solid.
Meant as an exploration of the unfettered masculine psyche from a feminine perspective, filtered through the prism of modern feminism. It's what Garland was dancing around much more elliptically with Ex Machina and Annihilation. Basically explained in the vicar's monologue before he was killed.
More or less nothing was real outside her husband dying and her friend showing up at the end after Harper (probably) killing someone.