555 TIMER CHIP IN MONOSTABLE MODE: You need 1/3 of the supply voltage source to "trigger" the TRIG pin, so 3V or less. To achieve that value the bottom left potentiometer needs its wiper >=84.4% (and the switch above that needs to be closed). Once the 555 is triggered, current flows through the OUT pin and the LED switches on. The resistance between the top-right potentiometer and the DIS pin will decide the length of the pulse for the LED. Higher wiper % = longer pulse. The voltage between the THR pin and the 15uF capacitor will decide the cut-off point of the OUT pin. Once voltage >= 2/3 (6V) of the supply voltage, then the 555 chip will stop current from going through the OUT pin and the LED will switch off. When the switch above the bottom left Potentiometer is open, if the voltage between the THR pin and the 15uF capacitor has not yet reached 6V, then the LED will stay on until such point is reached. So a quick ON/OFF of that switch will keep the LED on for a little while. With that switch closed for already a while, the voltage there will already have reached 6V and the OUT pin will cease to pass current through it and the LED will therefore switch off right away.
