The Lock Application button allows you the possibility to lock the application for security reasons when you're not in front of your computer.
The Lock Application button is only available when a password has been setup to open the application. You can setup an application password in File - Preferences - Security.
When a password has been setup, you can click on the Lock Application button to lock the application.
File - Lock Application
To unlock or open the application, the application password is required.
Enter application password