What are higher-kinded types in Scala and why are they important?

Higher-kinded types (HKT) are type constructors that take other type constructors as parameters — types of types. A regular generic like List[A] has kind * -> * (takes one concrete type, returns a concrete type). An HKT parameter is F[_] — F is a type constructor of kind * -> *. Example: trait Functor[F[_]] { def map[A, B](fa: F[A])(f: A => B): F[B] }. This Functor type class works for any F that is a type constructor — List, Option, Future, Either[E, ?], etc. Implement: implicit val listFunctor: Functor[List] = new Functor[List] { def map[A, B](fa: List[A])(f: A => B) = fa.map(f) }. HKTs enable writing code that is polymorphic over the container type — the same function works for List[Int], Option[Int], Future[Int]. This is the foundation of the Cats and Scalaz functional programming libraries. HKTs are one of Scala's key advantages over Java for functional programming.