By tracking your favorite Twitter accounts, we can inform you when they're about to reach a certain amount of followers or tweets, or when they've reached a milestone!
It's a good way to follow the growth of specific users.
To get notified on another Twitter account, search for any account you are interested in and simply click on the 'Track' button on the profile page.
To change the settings for the notifications go to Settings >>Tracking notifications make your desired changes and click the Save Settings button.