The details of the changes in the firmware were revealed by Nokia Developer in a blog post a fortnight ago:
As part of Nokia’s continuous development practice, we will be introducing new software updates for Nokia Lumia 920 and Nokia Lumia 820. This software, version 1232.5957.1308.00xx, includes further performance and stability improvements, enhancements to display stability during a call, and improved automatic display brightness adjustment.
In addition the new firmware enables a number of features necessary for other, optional, app-based software updates. The most noteworthy of these is the addition of a "storage check" module in the Settings app, which we explained in an earlier post:
Storage check, which is specific to Nokia Lumia devices, provides a more detailed breakdown of a phone's internal memory than the existing standard phone storage module. It also allow for users to manually delete temporary files, which should help alleviate the "other" storage issue that sees the the amount of space occupied by "other" files, as shown in the phone storage module, grow over time.
Availability
The 1308 firmware is now shipping on new Lumia 920, 820 and 620 devices. From today it is also available on the AT&T Lumia 920 variant. It will be made available to other existing devices in the coming weeks. The firmware is a Nokia specific update and is not tied to a Microsoft platform update. That means the OS version, 10211 (Windows Phone 8 GDR 1) will remain the same.
Once the software update becomes available your device will let you know about any update automatically, but you can also check manually using the phone update module of the Settings app. We'll also post a story once the software update starts to roll out.