Working of 2 input AND diode register?
+Vcc simply means the positive supply voltage. 

The inputs work because when you connect A OR B to 0V they pull the output Q down to a LOW voltage. (typically 0.7V for silicon diodes). The current that can flow through them is limited by the size of the resistor, R.

What the book didn't mention was that if the inputs are not connected to anything the output Q will also be HIGH.