b03-installing_RStudio_AC01.RmdR is a programming language, and when you download it there is also a basic GUI (graphical user interface) that you can access via the R icon.
A brief overview of installing RStudio can be found here “Getting RStudio on to your computer”
For a brief overview of RStudio see “Getting started with RStudio”
A good overview of what the different parts of RStudio can be seen in the image in this tweet: https://twitter.com/RLadiesNCL/status/1138812826917724160?s=20