HTTP URL on an HTTPS site
What it means
The site is served over HTTPS, but these internal URLs were crawled over plain HTTP. Even when they redirect, they leak the request to the network and cost an extra round trip.
How to fix it
Update the internal links to use https:// directly. Keep a redirect from HTTP to HTTPS across the whole site for outside traffic, and add an HSTS header (Strict-Transport-Security) so browsers stop trying HTTP at all after their first visit.
Finding it in CrawlStat
After a crawl, open Issues and pick HTTP URL on an HTTPS site. Click Show all in Pages to filter the Pages table to the affected pages, or Export affected to hand the list to whoever owns the fix. Audit ID: http_url_on_https_site.