HPC systems are usually accessed via a Linux-based Command Line Interface (CLI) that is provided by a shell.
At its core, a shell is simply a convenient tool that you can use to execute commands on a Linux computer. The shell provides a textual interface allowing to interact with the operating system and performing all possible operations, i.e., accessing and manipulating files, and running programs. However, there are various misconceptions that new users typically face when handling a shell such as the Bash. Particularly, dealing with control characters and the format expected when executing programs with arguments can be error-prone.
Part of this skill is the general principles of the interaction with a shell, to execute and to stop programs.