That's PR talk for lots of bug squishing. Indeed there are a fair few tweaks and problems the team tackled in the patch. Gamers should be wary of reading the patch notes; spoilers lurk within.
"This day one update is designed to optimize your player experience and is highly recommended," posted Ubisoft before offering a detailed breakdown in their patch notes.
Some of the fixes include smashing "a bug where counter-killing with the bow does not always work." Yes, getting that fixed would certainly work towards 'optimizing our play experience'. Another was that the camera "could remain stuck" after a player death, or even the player could get into a jam "after diving from a waterfall."
The patch notes are available on the official Ubisoft forums - beware spoilers! While the PC version must wait until November 20th to release, PC gamers do at least get to benefit from patches until then.