An XML Sitemap is an extremely useful tool in an SEO's arsenal. It allows you to unambiguously declare exactly which URLs you wish to be indexed on a website.
And this is also the only URLs an XML Sitemap should contain - only the URLs you actually want indexed.
As a minimum, these must be URLs that return a 200 status code, and are indexable. As such, the XML Sitemap Hints serve to highlight instances where this is not the case.
Most of the XML Sitemaps Hints are Issues, which means they represent errors or problems that need to be fixed. They are additionally classified in terms of their importance - this should be taken into account when prioritizing implementation work, along with the number and type of URLs affected.