TRIAC is a device that will acts as relay, Semiconductor Switch. It can handle 400V AC 16Amps (BTA16) More than enough. It works on basic principle, LDR when Light is more the resistance of LDR becomes low, so the charging of capacitor voltage is not enough to trigger the TRIAC. So it remains off during day time. At night LDR resistance become high and allows capacitor to charge above TRAIC triggering voltage threshold so Lights are on at night.
Saves energy....Low cost only 5 components...easy to install