In the following circuit ,
When light is not incident on the LDR , it acts as a very high resistance . Due to which the voltage at the base of the transistor is too LOW to turn the transistor ON.
No current flows through the emitter collector jn of the transistor and so NO CURRENT FLOWS THROUGH THE LED.
When light is incident on the LDR , it starts acting as a low resistance . Now , the voltage at the base is HIGH enough to turn the transistor ON.
The transistor starts working and current flows through the emitter collector junction .
Hence , the current flows through the led .
The pot or potentiometer is used to vary the resistance . So to find a cut-off point where the transistor just starts working .