RFID cards have a coil of wire in them and a radio transmitter. The reader induces a current in the wire coil which then momentarily powers the radio transmitter. This sends a 10 digit code to the RFID reader (and so to the Arduino). Each card has a unique number.
Therefore the software in the Arduino can work as follows:
"If I see number XXXXXYYYYY on the RFID reader, I must instruct the iPod to start playing track number X in the playlist."
It will do this every time it sees that particular card.
Later on I will show how the numbers are discovered for each card, it is best to just write the number on the back of each one.
Each card represents the first track of each album in the playlist, and the cover art from each CD is put on the front as a label.
Here are a few spare "bricks" on the floor waiting to be chosen next!