When your site's SSL certificate fails to encrypt all of the elements on a page (for example: forms, textboxes, images, etc.), visitor will receive an "unsecured content" pop-up message.
Mozilla Firefox displays:
“The connection to this website is not fully secure because it contains unencrypted elements (such as images).”
Microsoft Internet Explorer displays:
“Do you want to view only the webpage content that was delivered securely?
This webpage contains content that will not be delivered using a secure HTTPS connection, which could compromise the security of the entire webpage.”
Google Chrome displays:
“Your connection to example.com is encrypted with 256-bit encryption. However, this page includes other resources which are not secure. These resources can be viewed by others while in transit, and can be modified by an attacker to change the look of the page.”
This problem occurs if a web page contains hyperlinks to insecure elements. To resolve the error, all you need to do is find the elements on the page that cannot be encrypted and make some minor modifications.