Hreflang is interpreted by search engines and can be used by webmasters to clarify the lingual and geographical targeting of a website.
[1] Search engines use hreflang to understand the lingual and geographical targeting of websites and use the information to show the right URL in search results, depending on user language and region preference.
The hreflang definition is done by creating a full set of all language and region specific versions of the same document.
The x-default URL is not targeted at a specific region and/or language and is supposed to be shown to unspecified users.
Google suggests defining an x-default version in each URL set, which will be shown to users from unspecified regions or languages in search results.