Automatically saving changes into posts and pages is known as autosave. After the release of WordPress 3.6, it has been implemented into the post edit screen.
WordPress uses the user’s local storage feature of the browser to save the changes made by the user. This feature is very important because of internet connectivity issues. It prevents loss of data and saves to completed work automatically.
It is activated in WordPress by default. It is not required to add autosave features from third-party plugins.