When light falls on the LDR then its resistance decreases which results in increase of the voltage at pin 2 of the IC 555. IC 555 has got comparator inbuilt, which compares between the input voltage from pin2 and 1/3rd of the power supply voltage. When input falls below 1/3rd then output is set high otherwise it is set low. Since in
brightness, input voltage rises so we
obtain no positive voltage at output of pin 3 to drive relay or LED, besides in poor light condition we get output to energize.