Secure Sockets Layer (SSL)
SSL is the old protocol for encrypting the connection between a browser and a website. TLS replaced it, but the name stuck.
Secure Sockets Layer, or SSL, is the protocol that first encrypted the connection between a browser and a website, so nobody in between could read or change what passed along it. It has since been replaced by Transport Layer Security, or TLS, and every version of SSL is now retired as unsafe. The name lives on, though. People still say "SSL certificate" when they mean the certificate a site uses for TLS.
For a marketer, SSL is about trust you can see. A site with a working certificate loads over HTTPS and shows a padlock. A site without one gets a "Not secure" warning in the browser, which is enough to make many visitors leave before they read a word.
What a certificate does
A certificate proves your site is the one it claims to be, so the browser can set up an encrypted connection. Certificate authorities issue them, and many hosts now add one for free and renew it on their own, often through Let's Encrypt, which explains the process on its how it works page.
Certificates expire. When one lapses, browsers show a full-page warning instead of your site. For example, if Harrow & Pine's certificate runs out on a Friday night, anyone clicking through from an ad or a search result all weekend meets an error page, not a sofa.
Does it matter for search?
Google uses HTTPS as part of its page experience signals. It's a small factor, and almost every site has it now, so the risk is in breaking it rather than in not having it.
The same goes for AI platforms. A bot that can't make a secure connection to your page can't read it, so an expired certificate or a broken redirect from HTTP to HTTPS can quietly keep your pages out of the answers. Check that every version of your address ends up at one working HTTPS page.