This action lets you call static methods in classes. It is a more advanced action that requires you to know the full class name, method name, parameters and return type. You can generally find this info in the Unity scripting reference or the API reference for the class. A future iteration on this action will add dropdown menus and shortcuts to help you fill out this info.
The attached image shows an example, calling UnityEngine.Microphone.Start to record an AudioClip.
EDIT: Just noticed that Jean made custom microphone actions a while back
But the idea is the same, you can now call static methods without having to make custom actions...