When the 555 IC is used to produce an ASTABLE circuit - it will continually pulse until power is removed. Astable circuits can be used to flash lights/LEDs on and off or to turn a buzzer on and off repeatedly. They are also used in many more school based circuits.
Look at the circuit drawn below. Pins 6 and 2 are connected and go to the negative (0 volts). This is the easiest way of recognising that a 555 IC has been set up as astable.

An astable circuit such as the one seen below is sometimes called an oscillator as it resets itself, continually turning the LED on and off.

In an astable 555 circuit pins 2 and 6 are connected which means that the circuit will trigger itself continually until power is removed. The larger the value of the capacitor the longer the LED stays on and off.

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