How the pin2 of 555 timer triggers this simple circuit?
Three things to note:


Your body can be modeled as a resistor.
The touchpad is likely made up of 2 pads; one connected to pin 2 of the 555 IC and the other connected to ground.
The 555 is triggered by taking pin 2 lower than 1/3 of the supply voltage.


Pin 2 is being pulled up by R3 which is 1M ohms. This means that it is extremely weak. When your finger makes contact with both pads of the touchpad, the circuit sees essentially a resistor between pin 2 and ground.

Using the voltage divider equation:
$$
1/3 = R_{finger}/(R_{finger}+R_3)
$$
$$
\therefore R_{finger} = 500K
$$

The resistance provided by your finger needs to be less than 500K ohms, which is quite achievable if the 2 pads are close together.