first of all, the 100k acts as a pull up resistor, so that was why u were able to change it. the higher the resistance, the more sensitve the circuit becomes.
Your sensor sends gnd to the output when it receives enough light, which means if an led is connected between the output and gnd it will not turn on
But if the light received by the sensor is low, the resistance of the sensor becomes high and the led turns on. it is safer you use the ouput to switch the led.
this circuit can also be applied in street lights controle, with few components added