This is a quite simple touch sensor switch that’s build with the 555 timer IC.
You just must touch the metal plate and the relay gets energised and is kept on this state for about 100 seconds, then is released.
This sort of sensor switch is suitable for making touch operated bells, buzzers of small toys which operate for a small time and then switch off automatically.
The input impedance of the trigger is very high and the touch sensor switch might be triggered by the voltage induced in the human body. The relay is a 12 V DC relay operating at currents lower than 200 mA.