Modify screen in module programming

In this example we will write a module program with dynamic screen:
Business requirement:
The screen as below :

After user press the change button,the text field Depart.city,Arrival city,Flight time,Departure,Arrival Time will editable as below:

Here the code of the module pool program:

Notice that,to set read only these text fields,first make them in the group MOD

After pressing change button the code as below will execute:

Module programming(user dialog programming)

1.What is module programming(dialog programming) ?
2.Module pool programming step by step