SSL
Acronymtls, transport-layer-security, ssl certificate, https certificateDefinition
Secure Sockets Layer — the name everyone still uses for TLS (Transport Layer Security), the protocol that encrypts the connection between a browser and a server and proves, with a certificate signed by an authority the browser trusts, that the server is the one the name says; HTTPS is HTTP over it
SSL itself has been retired since 2015 (TLS 1.2 and 1.3 are what runs; the IETF specifies them), but the word survived in 'SSL certificate' and every hosting dashboard. What changed the economics was Let's Encrypt in 2016 — free, automated, ninety-day certificates — after which there is no reason for a page to be served plain. Cloudflare and Railway both issue and renew the certificate for this site's names without a step; the failure mode is a certificate for the wrong name or an expired one, and the browser's interstitial is unambiguous about either. It is why the handoff for designcoder.net named HTTPS as the first deliverable, not a feature.
The padlock is the only security UI a visitor reads, so it is the one thing a site cannot afford to get wrong. Mixed content — one image over plain HTTP on an HTTPS page — takes it away.