You can set the traffic limit (in Mbytes) by clicking the options, after you reached the traffic limit your mobile internet connection will be disabled.
So don't worry, you will not get any further charges from your provider. This app is running in the backround.
On the screen you will see the current uploaded, downloaded data in Mbytes and the download speed.
The application stores the download/upload data and the traffic limit. Your can reset these data when you want.
We hope you enjoy it! :)