If you bought MacBook anytime after February 2006, you may have been plagued by random battery issues. The issues, ranging from the battery not being recognized by the OS to not charging, or having low charge despite being fairly new. Many of these issues, it seems, are software-related and as such Apple has released a patch. To inform people specifically about the update pertaining to the battery patch, Apple has posted a webpage. All of these issues, Apple claims, are not safety related and don't pose any danger. Apple has also posted a disclaimer stating that if the patch doesn't address the issues for you, the battery will be replaced free of charge (assuming it is eligible).
Assuming you have software updates enabled in OS X, you can already download and install it automatically. Otherwise you can always download it as a standalone file from their site.