It’s a network protocol, like HTTP/S. It was inspired by the Gopher protocol from the '90s, but it supports basic text formatting (with Markdown-like syntax) and forces TLS encryption.
Please note that you will need a Gemini browser to view these sites. I recommend Lagrange, which is cross-platform, FOSS, and supports Gopher, Gemini, Finger, and Spartan; but other software is available.