Monostable 555: initially thr pin 6 is above the 2/3 vcc meaning output of SR latch is HIGH ( but LED is OFF due to inverter) turning discharge transistor ON allowing discharging of the capacitor. When the trigger pin 2 is pulled down by the push button to ground the output of SR latch is LOW ( but LED is ON due to inverter) turning the discharging transistor off allowing the charging of capacitor to 2/3 vcc turning the discharge transistor ON and turning OFF the LED.
