What is TLS certificate?

Also called SSL certificate, HTTPS certificate.

A TLS certificate is the file that proves your website is really yours and encrypts the connection to it. It is what puts HTTPS and the padlock in the address bar, and it expires on a fixed date.

In more detail

Most are issued free by Let’s Encrypt and last 90 days, renewed automatically by your host. "SSL" is the older name for the same thing and is still what most people call it. When one expires, browsers show a full-page interstitial warning rather than the site.

Why it matters for your site

An expired certificate does not degrade the experience, it replaces it — every visitor sees a red warning telling them your site is unsafe, and most leave. Almost every case is a renewal job that silently stopped, so the fix is checking that automatic renewal is on rather than renewing by hand.