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.

Here's the homepage: gemini.circumlunar.space

Here's a Gemini browser I like: gmi.skyjake.fi/lagrange

Here's my personal Gemini capsule (the equivalent of a website): gemini://deafeningcreationearthquake.flounder.online

And here's an old Gopher hole, for comparison: gopher://gopher.linkerror.com:70/1/txt_index

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.

Sorry if I've made this sound confusing.

tl;dr: It's an alternative to HTTP.