The photoresistor used has a dark resistance in excess of 200 kΩ and under bright light, the resistance falls to 1 or 2 kΩ.
To convert this varying value of resistance into something we can measure on BBB's analog input, it need to be converted into a voltage between 0 and 1.8V.
The simplest way to do that is to combine it with a fixed resistor, in this case of 10 kΩ in an arrangement called a voltage divider.
If the photoresistor is brightly illuminated, then its resistance will fall so it will pull the voltage up closer to 1.8V. If, on the other hand, the photoresistor is in the dark, the resistance will increase, so the fixed resistor will dominate and the analog input will be pulled close to 0V.
The VDD_ADC1 pin (P9, pin 32) is a steady 1.8V designed to be used to provide a reference voltage for situations like this.