Discourse Math Editor is a plugin to enter math equations into Discourse in a more user friendly way. This editor helps you to create LaTeX. Please read on wikipedia about it if you dont know what it is: wikipedia
For more documentation on all the LateX commands that you can use please have a look here: mathlive docs
And please watch the video:
Follow Install a Plugin
how-to from the official Discourse Meta, using
git clone https://github.com/spirobel/discourse-matheditor.git
as the plugin command.
You also need to use the official math plugin together with this:
offical math plugin
I recommend using KateX instead of MathJax, its way faster (at least for me).