What is django app usage?
Apps are logical modules. One app can contain several models. Your project could have users and blog apps. users would have User and Group models, blog would have Post, Tag and PostTag models. 
Views within single app usually have same URL prefix and their own URL routing.
Within app all database migrations are executed consecutively whereas it's your responsibility to specify dependencies between migrations from different modules.
Try to keep logical bounds between apps as weak as possible.
I want to know if I've understand the main point of django app usage.
Every app has a models.py file which create tables in our database, correst?
For example I want to create a personal CMS. I should create an app, to create tables for my posts details, and should create an other app to creating tables for my users that want to sign up into my blog, in order to keep their username and password in the database, and I also can create an other app to create a separate tables to save other data..... Do I think correctly?! What are django apps exactly for?