What is Mixed content?

Mixed content is a secure HTTPS page loading something over insecure HTTP. Browsers block most of it outright, so the usual symptom is part of your page silently not working.

In more detail

It typically appears after a site moves to HTTPS and an old hard-coded http:// URL for an image, script or stylesheet survives the move. Scripts and stylesheets are blocked; images may load with a warning depending on the browser.

Why it matters for your site

It undermines the point of HTTPS — an attacker who can modify the insecure request can change what the page does — and it usually presents as a mysterious broken feature that works on your machine because of a cached copy.