When you create a page that allows a user to create and/or edit entries, you can use a special URL that pre-fills a particular entry’s data into the page for editing. This allows you to, for example, email a user with a link to allow them to edit one particular entry alone.
Such a link can be emailed automatically to users after they create an entry using a particular page. (The option for this is in the page content tab.) Alternatively, you can access the URL for a particular entry directly using the Link to edit in… column in the main interface.