What's VIM?

Vim is a Unix text editor that's included in Linux, BSD, and macOS. It's known for being fast and efficient, in part because it's a small application that can run in a terminal (although it also has a graphical interface), but mostly because it can be controlled entirely with the keyboard with no need for menus or a mouse. For instance, to insert text into a file, you press I and type. To navigate or to issue a command (such as Save, Backspace, Home, End, and so on), you press Esc on your keyboard and then press whatever key or key combination corresponds with the action you want to take. It's a very different way of editing text compared to what modern computer users expect, but it's the way Unix admins all over the world edit config files, changelogs, scripts, and more.
Vim is also commonly referred to as Vi because when it was written in the late 1970s, it was short for visual editor. Before Vi, few people even imagined that a computer could act as a sort of interactive typewriter: text files were edited with commands that would find a specific line and either insert or remove text; literally all text was manipulated with what amounted to a rudimentary version of find-and-replace menu. Vi was a breath of fresh air, enabling users to enter a screen session that showed them their entire file and allowed them to edit it live.