How do AirPort Express and wireless access point relate?
The article is about reverse engineering the AirPlay part of the Airport Express device. The AirPlay feature allows iTunes to stream songs (audio) to the unit and you can connect a set of speakers or an amplifier to it via the 3.5mm stereo jack.

However the protocol (and the key) AirPlay uses is not documented, and if I want to implement the same feature on other hardware (for example make my linux gateway computer have the same functionality as an AirPlay device), I have to reverse engineer the protocol (the language both devices speak over the network) and re-implement it. 

The interesting part is that AirPlay uses asymmetrical encryption to encrypt the audio stream, and the private key is located only on the device. By obtaining this key, the last part of the puzzle is found.

After I wrote this, I have found an article, which describes it (probably better) than me.