Do you remember the map, from few SKBs ago ?
As a reminder, map allows you to transform a List[A] into a List[B].
What will happen if B is a List as well ?
flatten allows you to turn a List[List[A]] into a List[A].
Try to experiment with other containers such as Option we saw in the past.
What happen if you do
val a: Option[Int] = Some(12)
val aa: Option[Option[Int]] = a.map(x => Some(x))
val flattenA: Option[Int] = aa.flatten
Reveal more information and clues