This app creates an local VPN interface that is able to block the network activity of applications that are not root. It does not ask for permissions that could be dangerous like Location and contacts, SMS, storage ,… You can therefore be sure that it will not connect with a remote server to access your private information. It is safe to use!
Since this application is built upon the VPN Framework that is part of Android OS, so if you enable it, you will not be able to make use of another VPN application simultaneously, and it could drain the battery.
Certain IM applications (Instant Messaging apps, such as Skype) might utilize Google Play services to receive messages from the outside even if the app does not have a network. Therefore, you might also have to disable “Google Play Services” to prevent the receiving of messages for IM apps.