Has outgoing hreflang annotations using relative URLs
This means that the URL in question has hreflang annotations that have at least one outgoing hreflang annotation which is referenced as a relative URL.
Why is this important?
Hreflang tags are interpreted by search engines as indexing instructions. Search engines can follow and understand hreflang instructions if they are valid, consistent and reciprocal.
Using relative URLs for hreflang increases the chances that something will go wrong in the future, even if the setup is valid right now.
If the alternate hreflang <link> tag ends up pointing at broken, or incorrect URLs, the hreflang will become inconsistent, and search engines are likely to ignore it.
What does the Hint check?
This Hint will trigger for any URL which has outgoing hreflang annotations, where at least one uses a relative URL.
Examples that trigger this Hint:
Consider the URL: https://example.com/page-a/en/
The Hint would trigger for this URL if it had hreflang using relative URLs;
How do you resolve this issue?
This is the sort of issue that doesn't cause you any problems...until it causes you problems. Perhaps, in the future you move to a different domain or folder structure, which might in turn break all your hreflang annotations, and you only notice when international traffic falls through the floor.
We would very strongly recommend you change all relative hreflang URLs to absolute URLs.