What does this report contain?
All HTML pages which return a Non 200 HTTP response code in the headers.
How can this report be used?
Search engines will ignore the body content of pages which do not return a 200 status, so these pages will not be indexed, and the links will not be followed. They should be reviewed to ensure they are all low value pages, and can be reduced by removing or updating internal links to point directly to indexable URLs.