Problem with understanding the exercise: C# Part 2 Inheritance exercise: Design a Stack

I’m having a problem understanding the exercise.
I got the idea of creating the class and methods
but I didn’t understand the concept of stack
and how to implement it.

What object do I “push” where do I push it to?
Do I need to create a List? or an array first?
How do I remove the first pushed stack
and where do I return it?

Attached my attempt at understanding how to do this exercise