from the website blurb:
3.Y … bases itself on a "What if..." scenario. What if instead of embracing the new play-style the authors of the game rejected it and the new edition was a push back to the old play-style instead?
3.Y aims to be that imagined push-back. Rather than being presented as a whole game in itself, it is presented as a simple fourteen page set of changes to the rules of the game. Applying these rules modifications (and they have been extensively play-tested) will take your 3e game and give it the feel of earlier editions with a minimal amount of changes.
from About This Supplement:
This supplement aims to turn the game into that revised version. It has been designed as a supplement to the first version of the SRD (or the original game books that were compiled into that SRD) and instead of making it more complex and adding more scope for such things as “System Mastery”, “Classes as Toolkits”, “Caster Dominance”, “CoDzilla” and so forth - all of which are things that were slightly present in the first SRD and that were emphasised in the revised version of the SRD - it simplifies the game and removes such things.
from So Why Is It Called “3.Y”?:
Since the 3rd edition and the v3.5 books share a lot of commonality, they are often referred to together in the community as “3.x”. This is not an official name for the edition of the game, but it is something the fans use.
Since the terms of the OGL forbid me from using the official name of the game or the name of the SRD, this supplement (and the modified version of the game arrived at when using this supplement with the original version of the SRD) is instead called “3.Y” as a pun on the “3.x” label.
from And What Is So Different About It?:
Playing the game using this supplement is around 95% similar to playing using just the SRD on which it was based. This is to be expected, since the amount of material in this supplement is tiny compared to the amount in the whole SRD. However, the 5% that this supplement changes is the crucial part. This includes changes to key rules to do with characters and levels, spell casting, and combat.