What is MVC?

MVC is architecture pattern. It’s divided in main 3 sections:

1. Model
2. View
3. Controller

      Model: The Model represents a set of classes that describe the business logic and provides the data to view.

View:  View is responsible for look and feel. It is only responsible for displaying the data that is received from the controller as the result.

Controller: controller is responsible for take the user request and process the user's data with the help of Model and passing the results back to the View.

What are Action methods in ASP.NET MVC?

All the public methods of a Controller class are called Action methods.
Action methods is responsible for accept the request from user and with the help of Model and passing the results back to the View. 
Action method with the following restrictions:

1. Action method must be public. It cannot be private or protected
2. Action method cannot be overloaded
3. Action method cannot be a static method.

Action method are return ActionResult.
If you want to a public action methods  are not accessible from the URL. You decorate with NONACTION attribute.
E.g.

[NonAction]
public void AddressInfo(){
// some code inside here
}

[NonAction]
public ActionResult Address(){
// some code inside here
return View();
}