A protocol is the set of rules governing how two systems, most relevantly here, a browser and a server, communicate and exchange data. It shows up at the very start of a URL, even though most browsers hide it from the address bar by default: in https://zeo.org/, "https" is the protocol.
HTTPS (secure HTTP, encrypted using SSL/TLS) has become the effective standard for the modern web, and for good reason beyond just security: Google confirmed HTTPS as a ranking signal back in 2014, and modern browsers actively flag plain HTTP sites as "not secure" in the address bar, which is enough on its own to make visitors hesitant to enter any information on the site. The move from HTTP to HTTPS requires an SSL certificate and, done carelessly, can introduce mixed-content warnings or broken internal links if not every resource on the page is updated to the secure version, which is why migrating protocols is treated with the same care as any other domain-level change rather than a simple toggle.
