This App interfaces with the Communicate Service. So whenever any app want to send an 'Email', if you have installed this app, the email will be delivered to the recipients from your email ID.
To start using this app, you need to configure certain details like your email provider's SMTP host address and port number. You should also configure the 'username' and 'password' if applicable. The password you set here is secure and cannot be accessed by any other apps other that this app.
After configuring you can even test if your configuration is successfully by sending a test email from the user interface