When a site visitor visits a multi-language site, this setting allows the editor to automatically display the site in the language that fits the language of their browser, so they do not have to manually select the language from the Language Selector Module.
How to enable Automatic Language Detection
To enable Automatic Language Detection, simply click the checkmark on the left side of the option and do not forget to press Save to keep the changes. Once enabled, the editor will automatically redirect the user to their own language based upon the browser language.
What if the site does not have a version in the preferred language?
If a visitor has as their preferred language one that is not supported by the website, than the default language shall be used.