Missing hreflang annotations
This means that the URL in question is not specified as a translation through hreflang annotations.
Why is this important?
This may be completely intentional, for example, in a situation where a page does not warrant a translation, or the content does not translate well to another language. In this case, it would make perfect sense for the URL to not be included in hreflang.
However, hreflang is notoriously difficult to get right, and one issue that arises is that some pages get missed off the hreflang cluster, even if translations exist.
This Hint makes simply makes you aware that these pages exist, so you can decide whether it was intentional or accidental.
What does the Hint check?
This Hint will trigger for any URL which is not referenced via hreflang on any page on the site (despite some other pages having hreflang).
Why is this Hint marked 'Potential Issue'?
This Hint is a 'Potential Issue', which means that it is unlikely to be affecting the site at the moment, but should be investigated as it could cause issues in the future.
It is not necessary for every URL to be translated, just because some others are.
If you find that the URLs missing hreflang actually should have it, you may need to arrange for new translations to be produced, and for the hreflang to get added for all missing pages.