Is there a way or feature that allows form fillers to begin filling out a form, save it, and come back to it in order to complete later?

The easiest way to do this is to allow users to submit a partially completed form and send them a link to edit their submission.  Here is how to do so:

https://www.jotform.com/help/40-How-to-Let-Users-Update-Their-Form-Submissions-at-a-Later-Date

This will allow users to fill out the form and then return to their specific submission at a later time to complete the rest.