What is the Apple A9 chip

Not sure you want to know.

That is simply the processor that they have chosen ti use in this phone.

Apple designs its own chips for iOS devices to maximize their performance, provide specialized functions and be very frugal in power consumption. The -0A9 model is the newest version available.

For a good read on the A9 chip, try http://www.anandtech.com/show/9686/the-apple-iphone-6s-and-iphone-6s-plus-review /2

it's apples system on a chip which include a arm based main cpu and a powervr based gpu and a motion cpu thingy

all apples system on a chip is called A and the number indicate which version when they make a new one it will have a higher number