The title of the application already says quite a lot... it mutes the camera shutter sound on GSM devices, so on all non xx30 devices.
How you use the app:
You launch it via the icon, then press Menu -> Add MenuItem then you go to your camera app.
Now when you want to mute the sound of the shutter you simply click Menu -> Mute! and after you have taken the fotos you click Menu -> Unmute! to turn on the sound again.
After each reboot you have to add the MenuItem to the Menu again.