if your browse around Jerry Black's Github page (already linked above) you should find an answer to what the script installs, as well as some helpful information on how to run it, with the various parameters ranging from "all" to the specific sections, if you would like to step through it. There is the readme text, as well as comments in the script itself. Studying these before you download/run it might be helpful.
As far as running the script, you will need to download it then rename it (replace the .txt extension with .sh). Your GUI file explorer program could do this. Then go to your terminal emulator and use your keyboard to interact. You will want to use the "cd" command (example only: "cd Downloads") to change your working directory to wherever you downloaded the script, and the "chmod" command (example only: "chmod u+x buildKstarsNightlyFromGit_Bullseye.sh") may be required to set the permissions on the script file to be executable.