Active Content is a resource which can run in the context of your page and moreover can alter the entire page. If the HTTPS page includes active content like scripts or stylesheets retrieved through regular, cleartext HTTP, then the connection is only partially encrypted. The unencrypted content is accessible to sniffers.
VMware Cloud Director
The API documentation page in VMware Cloud Director (VCD) includes content like scripts or stylesheets that were loaded over HTTP, even though the page itself is using HTTPS.