When you are answering responses in your device, all answers are automatically stored within your device. So when you leave the app, you are not going to lose any data.
Your Responses are sent to the master web account when you press the finish button at the end of your form. If you are offline, do not worry.
All data is stored locally and will be automatically sent when you are online (whether via your WiFi or mobile data connection). Files are not sent via the mobile data connection by default (and are waiting for a WiFi connection to avoid using your mobile bandwidth limits), but you can easily change this in your app settings if required.