This schematic diagram is a light-operated, remote-controlled solidstate switch circuit to handle lamp. While in darkness, the resistance of Light Dependent Resistor (LDR) is extremely hight, it shoots up to megohm range. As a result, the triac isn't going to have gate drive and therefore it doesn't conduct.
When LDR is lighted by way of a torch-light beam, the level of resistance of LDR immediately reduces (under 10Kohm). This leads the triac to conduct and turn on the lamp. Light received from your lamp (normal lamp in the room, not from the torch) will keep LDR’s resistance low. So, the lamp stays continually "on". When the lamp is ‘on’, it could be turned "off’ once again by disturbing the light falling on LDR, by either waving hand in front of it or by disturbing power source for the circuit for just a moment.
RFC applied right here could be built by winding about 15 turns of 18 SWG wire over an insulated ferrite rod.
Circuit title: 220V Light-Operated On/Off Switch. This kind of circuit also can be modified to make a alarm circuit for security purpose.