That sentence means that the out parameters allows List to be flexible.

In kotlin there are two main type of lists:

• immutable List<>
• mutable MutableList<>

List is covariant and therefore we can do the following:

val integers = listOf<Int>(1)
val number : List<Number> = integers

`MutableList is invariant (limited) so the following code will result in compile time error

val mutableIntegers = mutableListOf<Int>(1)
val mutableNumber : MutableList<Number> = mutableIntegers // error Type missmatch

Let me know if this helps!