These devices are precision timing circuits capable of producing accurate time delays or oscillation. In the time-delay or monostable mode of operation, the timed interval is controlled by a single external resistor and capacitor network. In the astable mode of operation, the frequency and duty cycle can be controlled independently with two external resistors and a single external capacitor.
Monostable Mode is great for creating time delays. In this mode an external trigger causes the 555 timer to output a pulse of an adjustable duration.
Astable Mode outputs an oscillating pulse signal/waveform. In this mode the output of the 555 timer is switching between high and low states at a tunable frequency and pulse width.