Apple recently came out with another update for version 9. And this update is to fix 3 main security Issues. The Issues actually make your phone vulnerable to hackers. They could access your contacts, call history, photos etc. This update is a fix for that.
In software terminology, this is essentially called a Zero Day Bug . It is called zero day because the developer or the owner of the software would not know about this vulnerability until it is publicly announced. And once announced or reported the team gets zero time to fix it. It means that they have to fix it ASAP.
There is also a similar update for the Mac PC’s. This update also keeps away the Jailbreakers of getting any chance to hack the phone in order to add Apps.
This most likely is going to be the last update from Apple for iOS 9.