Online companies breaking into foreign markets will often have multiple site versions to accommodate different regions, nationalities and languages. The problem is that when Google sees multiple versions of a page, it can flag it as duplicate content; putting the site at risk of a Panda hit. As a result, it is important to alert the search engines to the various multilingual and international landing pages that exist.
In order to help with this process, Google has introduced a new rel-alternate-hreflang annotation for international landing pages where webmasters can specify default versions of their landing page. Google has been trying to simplify the annotation process following confusion over the correct use of tags last year. As a result, they stopped recommending rel=canonical hreflang tags
The new x-default hreflang attribute value signals to Google algorithms that a particular page isn’t designed to target any specific language or location and is subsequently, the default page when no other page is better suited.
Image Source: Google Webmaster Central Blog
This tag will help Google and Yandex to understand redirects and changes made to international and multilingual sites. It will tell Google that this particular page is the landing page for all languages or for a specific region, helping them to serve the right page to searchers.