vi is an old editor, still in use, particularly for small, embedded systems. You use vi mainly just to set up configuration files on a system, or in the case of a mis-configured or crashed system, you just might have to use vi to repair things in some text file.
In fact, vi is so unlike any modern user interface that there's no easy explanation of how to use it. On the other hand, there are probably hundreds of claims of "tutorial" or even videos (like this one). If you've never seen vi, please watch the video. Then, read some tutorial:
Alternates to vi
Many Linux distros nowadays have a simpler and more intuitive text editor called nano, which is good for making small, fast changes to files on your own Linux machine. However, vi is still an OS standard that you should be somewhat familiar with.