What this means is that you can no longer upgrade or downgrade to iOS 8.4 using iTunes. Only iOS 8.4.1 and later will be available to customers.
As Mac Rumors has stated, “iPhone, iPad or iPod touch users that have jailbroken their device on iOS 8.4 should refrain from updating to iOS 8.4.1, as the latest software update breaks the iOS 8.4 untethered jailbreak released in June by Chinese hacking team TaiG.
“Chinese team Pangu demonstrated an iOS 8.4.1 untethered jailbreak (via iPhoneHacks) at the HackPwn2015 security conference last week, but the method has not been publicly released. With the public release of iOS 9 expected in less than a month, it is possible the team may hold off on exposing an exploit to Apple for a minor software update.”