/sci/, through MathJax, allows users to enhance their posts with mathematics-related formulas, fonts and styling.
MathJax is essentially a LaTeX interpreter. Readers entirely unfamiliar with LateX are directed to Grätzer's More Math Into Latex book, the overleaf online guide, the quora online guide or whatever else you find on google.
To start with, Latex commands go between [math] and [/math] tags or [eqn] and [/eqn] tags. As in, [math]2 + 2 \leq 4[/math] or [eqn]\int x \ dx = x^2/2 + C[/eqn].
The distinction is that [math] displays the equation on the same line you started typing it, but [eqn] displays it on the next one and then adds a line break.
You can look up the LaTeX behind someone's post by right clicking it and selecting "Show Math As Tex Commands".
You can check out how your post will look like with the Tex button on the posting box.
The preview box is weird. Don't trust it too much. In doubt, the parser prefers it when you put in spaces between your Latex commands.
Some 4chan archives also support MathJax for /sci/, noticeably fireden and archived.moe.
Macros like \newcommand don't work. For obvious reasons.
\color doesn't work either, because of shitposting.
[math] commands can span out multiple lines.
MathJax is client-side, which means that your ad blocker is liable to break it.
If you can't recall a command, try out Detexify