This page is used to confirm SSL security. Most common errors with WP posts/pages using SSL are pages that include references to images that use the prefix http:// instead of https://.
For example, consider this image:
If you look at the SSL “lock icon” next to the URL in the web browser, you should see a warning, saying the site is insecure. Edit this page, change the src= line from http:// to https:// and save the page. Refresh it in your browser, and you will now see that it is secure!
This means that posts and pages that include image references to http:// will always come up as insecure, because they load web content using an insecure channel. Also, under Settings -> General, both the WordPress URL and the Site Address need to be changed from http:// to https://. Once done, theme header images need to be verified to not include http://, as was done with this page, under GK Portfolio -> Site Identity -> Logo Image.
One way to see why a page is having trouble being fully SSL compliant is to look at the page source, and do a search for:
This looks for references to plain http://, and a hit here means that the page source reveals that it is loading images in an insecure fashion, hence the error.