Apple has been having problems with the solid state storage on 2012 MacBook Airs for a while now, so it's great to see they've released a firmware update, and replacement program, to address them. Here are the details from Apple:
Apple has discovered that a small percentage of flash storage drives in these MacBook Air models have an issue that may result in data loss. This update tests your drive and, in the majority of cases, installs new firmware to resolve the issue. If your drive cannot be updated, Apple will replace it, free of charge
I've had the replacement done on my 2013 MacBook Air, which was still under AppleCare, but I haven't heard of any widespread issues with that model yet. If you have a 2012 model, get to updating, and let me know how how it goes for you.