Nothing is the type that sits at the bottom of the entire Kotlin hierarchy. Nothing extends all Kotlin types, including Nothing?.

But, why do we need a Nothing and Nothing? types?

Nothing represents an expression that can't be executed (basically throwing exceptions):

val result: String = nullableCupcake?.eat() ?: throw RuntimeException() // equivalent to nullableCupcake!!.eat()

On one hand of the Elvis operator, we have a String. On the other hand, we have Nothing. Because the common type between String and Nothing is String (instead of Any), the value result is a String.

Nothing also has a special meaning for the compiler. Once a Nothing type is returned on an expression, the lines after that are marked as unreachable.

Nothing? is the type of a null value:

val x: Nothing? = null

val nullsList: List<Nothing?> = listOf(null)