Everything you do in this application is saved to localStorage. You can backup this data by downloading a JSON file. You can use this file to restore the data on any other device or machine.
When you clear the data it will be replaced by default data.
Data synchronisation can be done directly through a peer-to-peer connection. There is a server involved to establish connection between the two peers but data is not sent through that server. Your phone sends directly to your computer or vice versa.
If you really must have a password you can enable encryption. But use with care: since no servers are used there is no way to reset your password if you forget it.
By default Project Invoice does not send or receive any data. But cloud synchronisation can be convenient if you want to use this application on multiple machines and/or devices. If you authorise a cloud provider the application will check the cloud for newer data when it loads, and save to the cloud every time you save to localStorage.