A patch is an update released after a game launch that changes its code or data to fix bugs, adjust balance, improve performance, or add small features. Patches are identified by version numbers and described in patch notes.
Early console games shipped finished or not at all, because cartridges and discs could not be updated. Connected PCs changed that, and always online consoles completed it: a modern game is expected to change after release. Day one patches fix issues found after discs were printed, and later patches respond to player reports, crash data, and balance statistics the developer could not gather before launch.
Patch notes are a small genre of their own. Good notes list every fix and every numerical change, state what behaviour is intended, and separate bug fixes from balance decisions, because players plan builds around those numbers. Vague notes force communities to test and document changes themselves, a process called datamining and community testing that fills wikis within hours of a release.
Not every update is a patch. A patch corrects or adjusts the existing game, while DLC and expansions add new content to buy, and a hotfix is a small urgent patch applied quickly, sometimes server side without any download. Players who say a game was patched mean its behaviour changed under them, for better or worse, without them buying anything new.
In play"Read the patch notes before logging in, because Tuesday patch changed how armour works."