What does (arg: n) in the command prompt mean?

On Codecademy's [Command Line Course](https://www.codecademy.com/learn/learn-the-command-line) , when trying to use the keyboard shortcut Alt+Shift+# (which is supposed to comment the current line) in the command prompt, it switches the prompt from $ to (arg: 3) instead of adding a dash at the beginning of the line. Alt+Shift+@ will make it display (arg: 2) instead, etc. See the last line in the screenshot below. Before I hit Alt+Shift+# it was just $. The shortcut works fine on my machine. 1. What is this (arg: n) thing? 2. What do keyboard shortcuts depend on to work? Keyboard layout? OS distro? Terminal emulator? Default CLI? What?