Gemini is an internet protocol started in June 2019 that is similar to Gopher (an old internet protocol which existed before today's HTTP) or as they call it "The web, stripped right back to its essence". Gemini can be seen as a middle ground between the old gopher protocol of the early 1990s and the modern web (HTTP).
I have come to love protocols like these which let me read people's sayings with ease; no flashy animations, no eye paining fonts, just pure content and nothing else.
It's a place you can be if you don't want resource intensive things going on in your server and if you don't have anything to do with media streaming(of course you can put media files in it, but the files should be downloaded in order to view them). Since it is mostly text, it is very accessible for anyone with a computer. Even a potato device can access the information stored and shared using this protocol.
The pages of Gemini are called "capsules" (like "websites" on the web) named after the Gemini space capsule. These capsules are written on a format similar to Markdown with file extension ".gmi" or ".gemini".
Since gemini is simple unlike HTTP many browsers and engines are built for it. There are portals as well, which even lets you access the protocol from HTTP browsers as well. The best way to start exploring is from their official site https://gemini.circumlunar.space/ and then check this.