If you enable Kill Switch, it will constantly monitor your connection for any changes in status or IP address.
If it sees a change (e.g. your VPN connection fails or disconnects), it will instantly shut down your device or prevent specified apps from connecting to the Internet (e.g. torrent client) until the VPN connection is restored or you have closed the VPN app.
This feature automatically connects your device to a secure, encrypted VPN server whenever you connect to an unsecured WiFi connection.