What is a ‘mixed content’ warning?

Mixed content is when a page loads over a secure https connection but pulls in some of its pieces - an image, a script, a font - over the old, unencrypted http. The browser flags it because the page is no longer fully protected:

• Why it matters - an attacker can tamper with the insecure pieces even though the main page is encrypted, and a loaded http script can compromise the whole page.
• What you see - the padlock disappears or shows a warning, and browsers now block the riskier insecure resources outright, so parts of the page simply break.
• The usual cause - a hard-coded http link left behind after a site moved to HTTPS, often in a template, plugin or old blog post.

Find the http links in your page source and change them to https - the warning clears as soon as every piece loads securely.