A monostable circuit is a one-shot trigger circuit. The cricuit waits, in its stable state, until you press a button, walk up to a door or some other action. It reacts carrying out your desired intention: turning on a light, opening a door or whatever else you can imagine. Once the action is completed it reliably goes back into its waiting state.
There is an ideal in this interaction which applies equally to hardware and software: the user interface and hence the required interaction, is kept at a minimum. Said simply: it just does what you want it to do.