When people describe a story as Lovecraftian, they’re often referring to one specific theme that permeated his work: the theme of forbidden knowledge. A Lovecraftian story usually involves a character learning some secret truth that is too horrible to fully comprehend and is driven to some awful fate by the knowledge. Learning the truth is horrible, but being ignorant of the truth is equally horrible. There’s no escaping the horrors of the world.
It’s a powerful theme, but also a pretty wide-ranging theme. Most Lovecraftian stories take this idea at face value, wringing horror out of things that are supposedly unimaginable—fear of the unknown taken to the extreme. But there are more ideas to mine from this theme than the concept of confronting “unimaginable horrors.”