What is it?
RMarkdown is a type of document/file that combines word processing with R code and output.
Who is it for?
RMarkdown can be superior to writing documents that use statistical results (graphs, tables, statistics) because it does not require merging documents and it is "reproducible research". I introduce my students to RMarkdown either after they've learned to use R, or if they are going to be using R for a lot of their work.
Resources
Morphius told Neo that the only way to really understand The Matrix was to experience it for himself. It's similar with RMarkdown. Here is a video I made demonstrating the very basics of RMarkdown. Here is the formal resource page from the RStudio company, which provides much more information. This page has basic features. Lastly, here is a cheatsheet for RMarkdown. Recently Posit upgraded RMarkdown to Quarto, which is basically the same functionality. RMarkdown is still maintained, but new development and future support will be with Quarto. Syntax is nearly identical, with a few slight simplifications.