You need to read the information shown here. Note towards the bottom that pin 2 on the Uno is actually Interrupt 0 and that pin 3 is interrupt 1. Therefore, this code:
int physicalPin = 2;
attachInterrupt(digitalPinToInterrupt(physicalPin), count, CHANGE);
you are using pin 2 of the Arduino to activate INT0 (interrupt vector 0). So if you attach a switch to pin 2 (via a resistor) and press the switch, you will generate a INT0 interrupt on pin 2. At that instant, it will call the function you've named count.