Basically instead of just relying a 4-digit code to unlock your phone, Apple has decided to make things tougher by introducing a 6-digit passcode instead. This means that even if hackers were to try and run brute force attacks to guess your passcode, they will now have more possibilities to deal with which could take them longer.
According to Apple, this increases the possibilities from 10,000 combinations to a whopping 1 million possible combinations. In fact along with the longer passcode, users will also be able to setup a custom alphanumeric code that involves letters and numbers, making it all the more harder for hackers to guess what your passcode is.
iOS 9 is currently only available to developers but the good news is that thanks to Apple’s public beta program, iOS users can look forward to taking iOS 9 for a spin come July, with the final release of the software pegged for the fall of 2015, presumably alongside the new iPhones.