Transport Layer Security (TLS)
TLS is the protocol that encrypts the connection between a browser and a website. It's the S in HTTPS and the successor to SSL.
Transport Layer Security, or TLS, is the protocol that encrypts the connection between a browser and a website. It's what puts the S in HTTPS and the padlock in the address bar. It replaced SSL years ago, although many people still say "SSL" when they mean TLS. MDN has a clear introduction to TLS.
For a marketer, TLS is a basic you just need to have working. Browsers warn people away from sites without it, and forms that send personal details without it are a risk to your customers.
What it does
TLS does three things. It encrypts what passes between the browser and your server, so nobody in between can read it. It checks the server's certificate, so the browser knows it's talking to your site and not an impostor. And it detects whether anything was changed on the way.
Your host or CDN usually handles TLS for you, and renews the certificate on a schedule. The things that go wrong are a certificate that expires, a new subdomain without one, or old pages that still load over plain HTTP.
TLS and search
Google counts HTTPS as part of page experience. Almost every site that ranks has it, so it won't lift you above anyone. Losing it, though, can cost you visitors right away.
For example, Harrow & Pine launches a new subdomain for its trade catalogue and forgets the certificate. Visitors see a browser warning, trade buyers who follow a link from a supplier's site turn back, and bots that insist on a secure connection can't read the catalogue at all.
TLS and AI bots
AI platforms' bots fetch your pages like any other visitor. If the connection fails, the page isn't read and can't be cited. When a page that used to show up in answers suddenly stops, a broken certificate or redirect is worth ruling out early.