TeX and LaTeX (and its cousins) are commonly used to write academic papers, especially in science and engineering. The usual workflow is to write the source file (a text file) in a text editor, then run some software to convert it to a nicely-rendered paper (e.g. a PDF document).
Wouldn’t it be nice if you could do all that online, in a web browser, something like Google Drive with a rendering engine? It turns out you can! In fact, there are several options, including:
If you’re looking for a good LaTeX reference book, I recommend LaTeX: A Document Preparation System (2nd Edition) by Leslie Lamport.