Design a basic Validation form in Python (Tkinter + Mysql) -- 2

This code that we design is for helping students understand how to validate the data that is being entered into the database. Database will be in mysql (SQL dump to be provided by us) and form to be created in Python ( Tkinter). We need to design just one form. You are expected to provide proper explanation of each line in the code. A fresher student should be able to study the code with this explanation independently. Note: Once the user enters the data into any of the fields, as soon as the user tries to go to the next field, e need to validate the data entered, if, it is not correct,we should not allow the user to go to the next field unless he/she enters the correct data in the current field.