LaTeX is a typesetting system that creates beautiful scientific documents. You can still submit handwritten homeworks, but we recommend using LaTeX.
Here are some resources on LaTeX to help you get started, written by CS109 TA Benson Kung in Spring 2020. There are many excellent tutorials on LaTeX online as well, but we've compiled some of our own so you can see how mathmatical notation works.
Here is an excellent LaTeX cheat sheet that you can reference.