Your site was fine yesterday. This morning every browser is throwing a full-page security warning, monitoring is screaming, and nobody deployed anything. A certificate expired. That's it. One of the most common self-inflicted outages in software, and it happens to companies with entire infrastructure teams who should know better.
You depend on certificates whether or not you understand them. And when they break, they break loudly and totally. There's no graceful degradation. Every visitor sees a scary red page.
Quick naming note: everyone says "SSL certificate" but SSL is the retired protocol. The artifact is actually an X.509 certificate and the protocol is TLS. The name stuck out of habit.
A certificate is a signed statement that binds a public key to a domain name. Nothing more. It doesn't vouch for the operator's honesty or the quality of the site behind it (I covered that distinction in my HTTPS post, so I won't repeat it here).
The signature on that statement comes from a certificate authority the client already trusts. During the TLS handshake, the server presents this certificate and the client validates it before trusting anything else about the connection. That's where it fits in the process.
