When a form is saved a session ID is generated. The session ID is used to later retrieve the contents of the form so that the user can continue completing the form where they left off. A session ID is also generated if an error occurs during form testing to prevent you from having to re-enter the data on the form.
Session IDs are made up of 32 characters and look like this:
If you want to delete the session ID and start with a fresh form, you can do so from the debug console.