The Monostable Mode
Unlike the sstable Mode of the timer 555, the monostable mode does not continuously transition between high and low states. When triggered, under the monostable mode, the timer 555 will only output a high signal for a certain period and then transitions back to low. The trigger will need to be pressed again to produce the high signal. This behaviour is due to the fact that the trigger pin is not connected to a charging capacitor (like in astable) but is rather connected to a manual input (e.g. button).
Let's see how a monostable circuit works.