Gemini is a internet protocol started in June 2019 that is similar to Gopher 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 and https).
Unlike http(s) it is text only, that means it only supports information in the form of text following the UNIX philosophy (Make each program do one thing well).
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 text only, it is very accessable for anyone with a computer, even a potato computing device can access the information stored and shared using this protocol.
The pages of Gemini are called "capsules" named after the Gemini space capsule. These capsules are written on a format similar to markdown with file extension ".gmi".
Android : Ariane, Deedum
IOS : Elaho
Windows : GemiNaut
Terminal : Amfora, Astro
Mac : Castor