What is it?
The fundamental way of issuing a command to the computer is by typing it on the command line.
Interfaces (like point-and-click graphical interfaces, or audio commands, or even games controllers) bypass the command line because they are more convenient. But when you study computer science, you go to the machine: you must get familiar with the command line.
The commands your computer understands vary depending on what kind of computer it is (of course!) but the fundamentals of commands are the same.
Specifically, you use the command line to send commands to the operating system.