Websites using the secure version of http protocol i.e. https are secure. When the browser makes an HTTP request to these websites, the web server will encrypt the data before sending a response.
If the browser detects mixed content on the page of a website, it will display a red colored triangle to the left of the browser's URL field. This indicates that the SSL certificate has been not been implemented correctly on a server or the page is having insecure internal links.
The mixed content errors usually scare away the visitors. Visitors leaving the website on finding errors is not good for the website's search engine ranking. This also affects the website's revenue.
To fix this error, enter the page URL in the textbox you see above and click the check button. The free Crawl Center mixed content checker tool will find insecure attachments, images, links, stylesheets on the page and it will display the same in a table.
Now, edit the source code of the page or update the post content in the database table to make sure that all internal links on the page are using https protocol instead of http.
Also check: SSL Certificate Checker