It depends on the country. In Britain, the monarchy is a constitutional monarchy, where the power rests with the people and the monarchs hold more of a ceremonial role. In monarchies that still wield actual power, the power in a monarchy derives from a social contract between the monarch and the vassal. The monarch agrees to protect the vassal from invaders and from injuries to the vassal’s person/property. In return for protection, the vassal agrees to help defend the monarch/kingdom in times of attack, to enforce the monarch’s law and to support the maintenance and upkeep of the monarchy by paying taxes and providing soldiers for the monarch’s service. The Magna Carta is a formalized example of that social contract. The details of how monarchies distribute power vary from monarchy to monarchy and change over time and place. If you’re looking for details on a particular monarchy, you might look into the history of that area and time and possible take a look at the law codes of the day
Much of the first answer is correct, but the latter part actually refers to “manoralism” and not to a monarchy. If the monarchy is an “absolute monarchy”, then the power is not shared. There have been times in history when there was great contention between, in England for example, between the King and Parliament over just what the King could do on his own (e.g. James I and Charles I). This led to, at different times, the King’s dismissal of Parliament or the dismissal (i.e. deposing of) the King. At other times there was much more of a sharing of power, such as when William and Mary ruled England in a “limited monarchy”. The King could not suspend laws at his whim; Parliament met much more often and wielded much more power. The constitutional monarchy, as noted in the first answer, leaves little real power at all to the monarch.