Unfortunately there is no info along with the update that provides any details about it.
A quick search on Twitter does not reveal anything about its contents but this update has apparently been out for most of the day.
It is very likely that this update is similar to the Surface Pro firmware update that came out last week and it is simply preparing Surface RT devices for the release of Windows 8.1 later this week.
I have contacted Microsoft to ask about the specifics for this update and will bring you that info once I hear back from them.
Thanks to @Fullmetal1986 for the tip.
Update: I heard back from Microsoft and there is no any additional info available right now. I have also heard from a few Surface RT users who have applied this update and are experiencing Touch Cover issues. Although I am not having those same issues here I am keeping an eye out for more folks talking about it.
On a final note, it also appears this firmware update has only been offered to users with Windows 8.1 RT Preview installed on their devices.