In short, and for non-computer-geeks: SSL certificates are used for data encryption, to securely transmit private data over the internet. You probably know web URLs starting with “https” instead of “http”. OK, now, those secure web sites may run on a machine that use the Debian GNU/Linux operating system. Unfortunately, Debian had a problem with generating such SSL certificates. They were recently uncovered to not secure your private data anymore. Luckily, a fix came out quickly, but there are still many websites using the broken certificates.
You want to be alarmed whenever you stumble over a website that still uses one of those bad SSL certificates! You want the SSL Blacklist add-on for your Firefox web browser!