lrx hello 🙂 if you're talking about the "Add an image" button in the Discussion/Post Composer toolbar then that weird combo of symbols and letters is what we call "Markdown" syntax.
Flarum has an extension bundled in called "Markdown" which adds support for this special syntax using the Litedown TextFormatter plugin. I find it to be the predecessor to BBCode in some ways.
You can find out more about how to use the Markdown/Litedown syntax here, more specifically related to Flarum's Markdown extension 😁
Unfortunately, this discussion is specific towards people interested in the Flarum QA team which we are always glad to have new people aboard to help us out, if interested 😉
Edit: Thanks to the lovely @clarkwinkelmann for moving our posts to a new discussion 🥰