As it generally does following new iOS releases, Apple today stopped signing iOS 10.2. The move comes following the release of iOS 10.2.1 earlier this month, which included bug fixes and security patches. Apple is also currently beta testing iOS 10.3, which will likely be released to the public sometime this Spring.
This means that users will no longer be able to downgrade to iOS 10.2 and instead will be forced to remain on iOS 10.2.1.
Apple generally stops signing older iOS releases as a way to prevent users from downgrading to operating systems that may have security holes that more recent releases fixed. In most cases, this primarily affects jailbreak users the most, but there was never a jailbreak available for iOS 10.2.
iOS 10.2.1 is a minor release and includes bug fixes and security improvements. More on that here.
iOS 10.3, however, will be a bigger update, bringing new features like Find My AirPods, a new file system, and iCloud Analytics for improving Siri.