A lightweight markup language is a simple, human-readable language for formatting text. It’s easy to read and compatible with most text editors. Documents written in lightweight markup are usually then converted to things that are harder for people, but easier for computers, to read, like HTML. The most common ones that I’ve heard of people using are Markdown, R Markdown, and reStructured Text. I imagine that most people who do data analysis/exploratory visualization/data science use a markup language more often than they write in raw HTML.