If the reset pin 4 of any 555 drops below 700mV then that 555 no longer performs any task whilst in that state. The task in this circuit is to rapidly flash an LED using a typical astable setup. Connecting pin 4 to Vcc is often used just to keep it above 700mV out of the way, but it can be used for all sorts of things eg in this circuit....turn the variable resistor just below 700mV seen on pin 4 wire and see the LED stop. Then carefully move the variable resistor above 700mV and see the LED come on. Knowing this enables you to design your circuits using pin 4 as a switch. Take a look at my ' Thermistor fridge cold ' circuit where pin 4 wire is used as a thermistor...you could use it for an LDR light dark switch as part of your circuit etc.
