Has outgoing hreflang annotations using relative URLs issue means that the URL 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 tag ends up pointing at broken, or incorrect URLs, the hreflang will become inconsistent, and search engines are likely to ignore it.