The singer is eating blackberries when he spies a milkmaid. He asks to buy milk; she says the cow has kicked over the bucket. She hints that the loan of a shilling would be quickly repaid; he takes the hint, she takes the shillings, and he takes her
Not to be confused with "Pretty Betsy the Milkmaid (Blackberry Fold)," despite their sharing a milkmaid and blackberries. Incidentally, one of the reasons milkmaids were held in such romantic esteem was for their smooth, fair, and un-pockmarked skin, which came from their contact with cowpox and resultant immunity to smallpox. - PJS
Kennedy observes that the song dates itself to Michaelmas (September 29), a day on which hired workers finished their terms and were paid off. Thus the youth would have money to spend -- and the girl would have every reason to latch onto him *now* (even if it meant spilling the milk) before he left the vicinity.
I know of no version in which the two explicitly sleep together (and can't imagine Baring-Gould printing such!), but the implication is strong. - RBW