it depends on how you make the sensor. if the transistor is the higher side resistance, you'll get + when it's open (hit by light).
if it is the lower side (ground connected) resistance, when it will open, you'll get ground (0 V). In theory at least.
The function calls the Atmega's onboard ADC. This device transforms the analog voltage into a digital value which is then returned to your function, which in turn, does something with it.