Terminal is an application that comes on every Mac. It allows us to go inside the computer, and configure the way our OS is running. This application should never be used if you do not know what you are doing; because you can severely damage your OS software. You might be asking, what would be the reason the configure your OS, or how can this application be helpful?
Well, one reason that people will configure there OS, is because something they may want to change may not be available in the system preferences. By this I mean that the option is not available. For example, when you are in the log-in screen, you may not like the background image, the option to change it in the system preferences in not available. However, if you go into your Terminal application, and you know exactly how to access that file; you will have the ability to change that image just from this application.
Changing images is just a very small portion of what exactly you can do from terminal. There are many other things like editing files, editing applications, customizing the look of your computer. There are many things that you can do in Terminal.
You just have to be careful, if you do not know what you are doing, you might in fact permanently damage your computer. This means that if you are trying to edit a file, you may have broke your computer so it will not longer run.
So if you are the type of person to make your computer unique, I would recommend looking into this type of application. Just read up on what you want to do, because the consequences may turn out deadly.