The let-down reflex is the process through which the nerves in the breast send signals that release the milk in your milk ducts. In other words, it is what allows breastmilk to flow.
The nerves of your breast can be stimulated by your baby sucking, by hearing or seeing your baby, by using a breast pump or by touching your nipples. When the nerves are stimulated, oxytocin and prolactin are released into the blood stream.
While prolactin helps make the milk, oxytocin is responsible for making the milk available to baby. It does this by widening the milk ducts to make it easier for the milk to flow down. Cells around the alveoli then contract and squeeze out the milk, pushing it towards the nipple. Pretty clever, eh?