You've already encountered Vimscript when you worked on your .vimrc. What you may not have known is that Vimscript is actually a Turing-complete scripting language—there's no limit to what you can do. So far, you've used it to set variables and perform a few comparison operations, but it can do so much more!
You will learn how Vimscript not only helps you understand Vim configuration better, but lets you solve text editing problems you encounter by writing functions and plugins.
It's pretty awesome.