TeX is a popular typesetting system for authoring technical documents, and in particular, math expressions. There are many implementations of TeX such as LaTeX, a popular implementation used in academia for books and papers. This website uses KaTeX for typesetting math expressions.
As opposed to WYSIWYG (what you see is what you get) editors, TeX editors take in plain text as input and renders to the output. For example, the plain text below is passed to the typesetting system as input.
This input is then rendered to the following math expression.