SSL/TLS is used for secure communication between your browser and web sites. You can tell it's working when you see a lock next to the web address and the address itself has "https:" at the beginning instead of just "http:".
SSL/TLS relies on certificates issued by trusted authorities.
Unfortunately, some certificates are fake, or stolen.
3rdKey.com gets a list of bad SSL certificates from badssl.com, and checks them against your browser, and if we find any problems, we will let you know.
Of course we can't check everything because it's constantly changing, but we do try to keep up to date.
We use badssl.com for for checking your SSL.
We also recommend ssllabs.com.