External redirected URLs
This means that the URL in question is an external URL that redirects to another external URL.
Why is this important?
A redirected URL, typically 301 (permanent) or 302 (temporary) means that the location of the page has changed, and the user is sent from the original URL to a new one. Since content gets changed and moved all the time, it is relatively common for redirects to occur, however, they make the experience worse for users.
What does the Hint check?
This Hint will trigger for any external URL which returns a HTTP status code of 3XX.
Examples that trigger this Hint:
The Hint would trigger for any URL if it returned a 301 HTTP header response;
Or indeed any other 3XX HTTP response.
Why is this Hint marked 'Insight'?
This Hint is an 'Insight', which means there isn't necessarily any action that needs to be taken - the Hint is intended to alert your attention to something, rather than flagging up an issue that needs fixing.
In this case, it is not an SEO issue, simply a (relatively small) user issue. Whereas internal redirects can have an impact upon crawl budget and load speed, this does not apply to external redirects.
Considering how little control you have over external URLs - which can change all the time without your knowledge - we would consider this a low priority to deal with.