This error means the URL you list in your HREFLang element is redirecting to a URL that is different than the one that you listed in the HREFLang Tag.  For example 


The following are 3 common errors:  


Error Type #1 - No Trailing Slash or Added Trailing Slash

With this case the URL was listed without a trailing slash - when the spider or visitor goes to that URL the server redirects them to the version with a trailing / making it a totally different URL.  

<link rel="alternate" href="https://www.mysite.com/uk" hreflang="en-GB"/>


Error Type #2 - HREFLang Set to old HTTP version but redirected to HTTPS

With this case, the site may have migrated to HTTPS but the HREFLang is still referencing the HTTP version of the page.  When the spider follows the HREF URL they are 301 redirected to https which is totally different page.   


<link rel="alternate" href="http://www.mysite.com/us" hreflang="en-US"/>


Incorrect Example:  


In the image below, the BBC has referenced the URL to their Welsh page as http://www.bbc.com/cymru/ but if you go to that page, there is a 301 redirect to http://www.bbc.com/cymru which is the same page minus the trailing / and effectively a different page.  Accordingly, you would get a non-referencing error.  In addition, Google has to make an additional request for the correct URL both of which reduce your crawl budget unnecessarily.