The Insecure Content Report

Using http:// content in an https:// website often causes mixed content errors. But even when it doesn't, having insecure content is bad, pointing your users to insecure content elsewhere isn't great either.

The source URL is the url that the insecure content is on or linked from.

This will contain the html of the insecure content. If this is an img tag then likely your users will also be experiencing mixed content errors or warnings. If it is an a tag then it is something that has been linked to.

In both pages you should find a secure https: alternative and change it to that instead.