This book is a freeware variant on "Dungeon World", which in turn is part of the Apocalypse rule system. I haven't gotten to play this one yet but it looks mercifully simple and story-driven, with all of the necessary rules here but no specific setting. (That's meant to be sold separately, but you don't really need one just to fool around with a fantasy adventure.) It is somewhat simpler than the default DW rules, for instance because it ditches the vestigal "make D&D stats, derive modifiers, then mostly ignore the stats themselves" rule. Hit points are abstracted out, too. The rules in general are desgined around giving the players tough choices like, "Your fire breath hurts the enemies, doesn't hurt your friends, and doesn't cause collateral damage... pick two." Or even "pick one", if you roll poorly!
The character types are pretty similar to DW's, but with sillier descriptions (eg. Paladin becomes Shiny Paladin, with character traits like "fantasy armor with lots of crystals") and shorter, simpler sets of moves. There's also a rule system about gathering Clues and MacGuffins to defeat powerful foes, which is a substitute for the fact that nobody actually has hit points. So, again, the focus is on simplicity and storytelling. I see that as a good thing, mostly. It should be possible to pull in aspects of the full DW rules if you ever feel like adding their more complex features like the design of villain organizations.
This book seems like both a good intro to DW itself, and a fun system to play in its own right. Looking forward to either.