SSL/TLS is the cryptographic protocol that secures HTTPS connections. It encrypts traffic between browser and server, authenticates the server's identity via a certificate, and ensures message integrity. SSL is the deprecated name; modern deployments use TLS…
Each HTTPS site presents a certificate signed by a trusted Certificate Authority (CA). Browsers verify the signature chain and reject connections to expired, mismatched, or unsigned certificates with a full-screen warning that destroys conversion rate.
Certificates have expiry dates (typically 90 days for Let's Encrypt, 1-2 years for paid CAs). Forgetting to renew is one of the most common causes of preventable downtime — even at large companies.
An expired certificate makes a site appear broken to every visitor. Monitoring certificate expiry with multi-week lead time, plus alerting on TLS configuration regressions (weak ciphers, missing OCSP stapling), is essential for any production site.
See it in the product: SSL monitoring.