Grunt calls itself "The JavaScript Task Runner." Huh? What's a JavaScript task runner?

The JavaScript part comes from its dependency on using Node.js. The task runner is another way of saying automation. So what we have is an automation tool built on top of Node.js.

What can it do, and why do I need it?

As a Drupal themer/developer, I'm all about being lazy. Lazy in that I don't like doing the same thing over and over again. It compiles & watches for SASS changes. It LiveReloads. It lints, minifies & combines JavaScript and/or CSS files. It copies library assets. It can minify & combine images. Plus tons more. Pretty much any repetitive task can accomplished with Grunt.