ADChannel0/1/2/3

These variables return the voltage on pins A0/1/2/3 as a value in the range 0 - 1023.

This range of values represents a voltage range of 0 - 3.3 volts, or ~3.222 mV per step.

//convert channel 0 reading to millivolts
int voltage = ( ADChannel0 * 3222 ) / 1000 ;