[Jtsummers]

https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/type... <- This does a better job than I could at explaining it. But basically, certain expressions are "holes", incomplete/invalid code but where the type of the hole (a variable or _) can be determined. The compiler can also give suggestions on how to fill a hole.

I learned about them with Idris and Type-Driven Development by Brady, I've never used them in Haskell but haven't really touched it seriously in a decade now.