If you want to test an early version of the BeeTagg QR Reader (alpha or beta version), you will have to be comfortable with having test software put on your mobile phone. The most likely problem is that the app just won't work - which is one of the main things we are testing.
In that case, all your other apps should still work. In the unlikely case that a broken app causes problems with your other apps or operating system, it might mess up your personal data. In that case, you'll want to be careful to recover using the backup that your sync software makes when you sync your device with your PC.
In an extreme case you might have to to restore your phone to its original settings, in which case you might lose your data. We think that this is very unlikely, but you should be aware that you are taking a risk.