About 16 million homeowners owe more on their mortgage than their homes are worth, which means they are "underwater." So long as that condition continues, they have no equity that can be used to help finance the purchase of another house. On the contrary, they can’t sell the house without digging into their pockets to pay the difference between what they owe and what they can realize from the sale net of expenses.
But time heals most wounds, and negative equity is no exception. The principal component of the monthly mortgage payment reduces the loan balance by the same amount. Refinancing into a mortgage carrying a lower interest rate reduces the interest portion of the monthly mortgage payment, thereby increasing the principal component and the rate at which the balance is paid down.