How do you put a List under an Array item (Hard time explaining)

Hello all!

Looking for how to do something. I want to have an array (public int stage) but under each stage I want to have a List of monster prefabs
In the Unity editor that is. So to put an array under the stage int.

I’m not sure what this is called. I want something in the Inspector to show:

[Stages]

You click on stages then it has an array, each one added has a stage number and a list of game object mobs.

Argh… It was as simple as adding [System.Serializable] to my class. Thanks.