Grunt

Basically two files Gruntfile.js & package.json

I encourage you to read Grunt's Get Started guide to familiarize yourself with grunt's workflow

package.json

Holds the needed packages names and versions, mainly packages for CSS & JavaScript minifying, Sass ruby compiler interface for NodeJS and localization file generator

Gruntfile.js

Loading and implementing each task, the main section of focus would be the plugin_args variable which holds general arguments used for each task setup.

watch:all is the default task when grunt command line runs.

Note: when the watch task runs every time a last_update file generates/updates inside the assets folder with the date & time of the change which is useful for using it as version while loading the assets wp_enqueue_script, wp_enqueue_style, etc.