What is SSL? Understanding the padlock in your browser

SSL (Secure Sockets Layer) is the original protocol for encrypting traffic between a browser and a server — it's technically been replaced by its successor, TLS (Transport Layer Security), but 'SSL' stuck around as the everyday term people still use for both, including in 'SSL certificate'.

An SSL/TLS certificate does two jobs at once. First, it encrypts the connection, so anyone intercepting the traffic between you and the website — on public Wi-Fi, for instance — sees scrambled data instead of your password or credit card number. Second, it verifies identity: the certificate is issued by a trusted Certificate Authority that checked the site actually controls the domain it claims to be, which is why a valid certificate turns into a padlock icon instead of a warning.

Certificates aren't permanent — they have a validity window (commonly 90 days for free certificates from Let's Encrypt, up to a year or more for others) and need to be renewed before they expire. An expired certificate causes the browser to show a security warning even if nothing else about the site changed, which is why site owners regularly check their certificate's expiration date rather than waiting for a user complaint.

You can check any site's certificate — issuer, validity dates, days remaining — with Tool King's SSL Checker, without needing to dig through your browser's developer tools.