This is an Arduino theremin that is based on light intensity. It's a simplified version of the original theremin which is based on electromagnetics and where you can control both amplitude and frequency.
But it’s a fun and simple project anyway!
The photoresistor is connected with the resistor to form a voltage divider. The Arduino reads the voltage out from the voltage divider.
This means that when the photoresistor changes its resistance (that is when the light changes) the voltage that the Arduino reads changes.
The Arduino controls the frequency of the buzzer. By using the voltage value the Arduino reads in, the Arduino changes the tone of the buzzer so that the tone you hear is directly dependent on the light that the photoresistor sees.