The Shell or commandline is an interface to interact with a given computer (or supercomputer). Contrary to the graphical user interface most desktop machines use, the shell is very basic and requires the user to have a certain knowledge on what commands there are and how to use them. However, even with a few basic commands, the shell can be a very powerful tool. Combining this power with the fact that it almost uses 0 resources is the reason, why it is still in use on (nearly) every (super)computer.
Enter a command, hit return and read/wait for the answer :P