Typeset input is entered into Discourse using LaTeX snippets which is then rendering using a JavaScript program called MathJax. For example,
$$\int_{-\infty}^{\infty} e^{-x^2} dx = \sqrt{\pi}$$
is entered as follows:
$$\int_{-\infty}^{\infty} e^{-x^2} dx = \sqrt{\pi}$$Note that the double dollar signs ($$) are used to delimit the LaTeX input; everything between the double dollar signs are the LaTeX command that lead to the typeset expression. Use of double dollar signs indicate that the expression should be set on it's own line in display mode. You can use single dollar signs to include a simple math expression inline. For example, $\sqrt{\pi}$ by itself is input as
\$\sqrt{\pi}\$LaTeX itself is a huge system that can be used to create large documents - but you need only use small snippets for this site. This is actually a gentle way to get started with this valuable tool, anyway! There is plenty of information on LaTeX available online. A great tutorial for using the MathJax version of LaTeX is available here. Of course, you'll learn as you go along and one useful technique is to hit the edit button on a post like this one so that you can see the code as entered.
