Autosave is the feature in WordPress which automatically saves the changes you made in posts and pages. Changes you made are automatically saved in every few seconds.
Autosave option is automatically enabled in every post and pages and this feature can’t be disabled.
When internet connectivity is lost during the content modification, WordPress can’t access your database to store the auto-saved content. In this case, WordPress takes the advantage of local storage feature in the browser to save the changes that made in the post. Now changes you made are stored in browsers local storage and can restore the backup of stored content once internet connection was established.