I believe the commenter may have been talking about what I've seen called fix-point recursion, and it is exactly as you say, it is for representing arbitrarily nested tree structures in a statically typed way.

For example in scala, maybe you have data about Users

  case class User(id: Long, name: String, friends: List[User])

However, this structures is kinda inflexible, you don't always want to deal with an entire tree of users at a time. Maybe, you just want to have the friend userids, so you can introduce a type parameter for the recursive data

  case class User[Friend](id: Long, name: String, friends: List[Friend])

and then you can have a User[Long] which contains the friend ids, or User[String] which contains the friend names. However, if you want to go back and have the list be of Users themselves, you would have to know statically how deep the tree you're returning is. For example, this type would include full user information down to the grandchild level, but then terminate with the ids:

  User[User[User[Long]]]

This is actually very useful! Sometimes we may want exactly this deep of a structure. If we want to get an arbitrarily deep tree of users, we would need to have an infinitely nested type like

  User[User[User[User[User[User[...]]]]]]

which is of course impossible. So instead, people have a work-around using higher-kinded types, called `Fix`:

  case class Fix[F[_]](unfix: F[Fix[F]])

This signature may look scary, but if you apply it to User you get

  Fix[User](unfix: User[Fix[User]])

and all this means is that now we can deal with Fix[User] which means the same thing as User[User[...]]. You can construct one such Fix[User] differing levels like so:

  Fix(
    User(
      id = 1L,
      name = "Josh",
      friends = List(
        Fix(
          User(
            id = 2L,
            name = "John",
            friends = List(
              Fix(
                User(
                  id = 3L,
                  name = "Stacey"
                  friends = List.empty
                )
              )
          )
        ),
  
        Fix(
          User(
            id = 4L
            name = "Mary"
            friends = List.empty
          )
        )
    )
  )