We love this feature: when enabled, this will periodically auto-save the form data as your user fills it. So, if your user closes the browser without submitting the form, and comes back to the form some other time, he will find the form data waiting for him. There is no manual save button, as the form data is auto-saved periodically on the user’s PC.
PS: If you want the user to know about this, you add a custom text field in the form letting him know about this.
Enable Auto-Saving Form Progress
In the form builder, go to Form Options -> General