As a layman I would describe them as implicits like in current Scala [0] but better (easier to reason about). One thing which they allow is ad-hoc polymorphism like typeclasses.

https://arxiv.org/pdf/1512.01895.pdf

https://old.reddit.com/r/ocaml/comments/4qan0w/modular_type_...

[0] https://scala-lang.org