"Mixed content" means a secure page is loading some items over insecure http://.

1. The cause is usually images, scripts, or stylesheets hard-coded with http:// URLs.
2. WordPress: a plugin like Really Simple SSL rewrites these to https:// automatically. Or use a search-and-replace plugin to update old http://yourdomain.co.za links to https://.
3. Any site: edit the page/template and change http:// links to https:// (or protocol-relative //).
4. Use your browser's developer console to see exactly which items are insecure.

Tip: After fixing, reload with the cache cleared. The padlock should appear without a warning.