According to the brief release notes, Apple's MacBook Air SMC Update version 2.0 targets a power handling issue that appears to be limited to mid-2013 MacBook Air models.
Specifically, the update addresses an issue that "may cause the battery to drain faster than expected when the lid is closed." A number of threads on Apple's Support Communities forum attested to the battery drain problem, with one of the larger discussions suggesting the issue is OS-related.
Last October, Apple issued MacBook Air SMC Update version 1.9, which addressed a serious issue where the battery would unexpectedly stop functioning or would not be recognized by the system.
Apple's MacBook Air SMC update requires OS X Mavericks 10.9.2 or later, or OS X Mountain Lion 10.8.5 or later to run. The download comes in at 804KB and is available from Apple's support website.