Shylock has had a dream, or rather a nightmare, involving moneybags. To him, they represent a bad omen, a premonition that something terrible is about to happen. As Shylock's moneylending business is the most important thing in his life, we can safely assume that he's worried about the potential loss of his trade.

Despite Launcelot's gentle ribbing, Shylock takes his dream very seriously indeed. So much so that in act 2, scene 5, he orders his daughter Jessica to stay indoors that evening while he's out dining with Bassanio and not to allow any merriment in the house. There must be no bawdy Christian masques at Shylock's place.

But Shylock is right to be worried; Jessica's planning to elope with Lorenzo. Yet it is his material possessions, his house and his money, that he's afraid of losing, when in fact it's his daughter he's about to lose. This is a classic example of dramatic irony, where we know something that a character doesn't.