The black dot at the back of your iPhone, between the rear camera and flash, is a tertiary microphone.
So, an iPhone in general has three microphones: one is near the speakers on the bottom, second is located near the earpiece and the third is at the back, near the camera.
A piece of brilliant engineering, that black microphone is a powerful noise cancellation object. When you're making an HD video or FaceTiming, Skyping and most importantly when you're answering an important call and are surrounded by a chaotic background, this microphone digitally eliminates the ambient background noise and enables the caller listen to your voice clearly and without any interruptions.
Technically, it's not a microphone because it DOESN'T listen to your voice and so it's called a noise cancelling microphone.
Stay tuned in for more such know hows and whys!
Title image: Digitaltrends