Physiologically, regulation of milk production and let-down during lactation is regulated by the hormones prolactin and oxytocin together with neuronal feedback loops caused by baby sucking on mum's nipple.
Without going into too much detail, sucking by baby stimulates oxytocin release which affects myoepithelial cells. Contraction of these cells moves milk forward in the ductal system of the breasts. Milk let down promotes prolactin release which affects the mammary glands and causes them to produce more milk.
If demand breastfeeding does not occur or stops (as in weaning), the feedback loop stops and the prolactin priming of the mammary glands stops and they will, over some time, return to their pre-lactation state (more or less).