Rockstar haven't yet added this to the official site yet, so we're not sure of the exact details of what it does. However, some players are reporting performance increases with an extra 5 fps, sometimes even more. Guess it all depends on your setup. Let us know in the comments how the patch effects your game. We'll post full details of the changelog once Rockstar put it up.
UPDATE: It's simply a maintenance update:
* -Resource management adjustments based on the operating system detected.
As an aside: Both ATi and nVidia this week released updates to their graphics card drivers. These can of course also help give you better performing games, so it's recommended you upgrade these as well.
Links: ATi Catalyst 9.6 or nVidia 186.18 for the latest drivers.
EDIT: Local mirror of the patch here: download patch 126.96.36.199