Indie game storeFree gamesFun gamesHorror games
Game developmentAssetsComics

I'm having trouble understanding what this is, is it a tutorial or a tool?

Could I use this to define hitboxes for a spritesheet and output a data file for my non-GameMaker engine?

This is a bunch of code that you can use to learn how to do hitboxes and hurtboxes in an easy to manage way.

so it's a tutorial, then. Thanks for the speedy answer