According to the release notes, Apple's firmware update 7.7.2 applies to AirPort Extreme and Time Capsule Wi-Fi base stations that support the 802.11ac protocol.
The most pressing issue fixed with the update is a bug that prevented USB hard drives connected to an AirPort from appearing as an available disk in Time Machine. Apple's base stations allow users to connect external hard drives to maximize storage space, or make more storage available wirelessly.
Also addressed is an issue that caused intermittent WAN outages for some users, and another that resulted in a network loop.
Apple's firmware update 7.7.2 is available for download now from Apple's Support Downloads webpage.