[NOTE: the section on bonds described here is not yet implemented; for now a debt simply means negative money]
If your nation's money balance goes negative, you will be forced to issue bonds to your population to finance the debt (this happens automatically over the update, so there is no way you can plummet into a negative balance). You must then pay an interest on these bonds.
At any time you can also negotiate for other nations to purchase your bonds. Interest rate on domestic bonds is fixed (15can negotiate the price for bonds issued to other countries.
If you want to finance a big war, and get lots of cash fast, the best way to go is probably to issue a lot of bonds to other countries.
(??) The bonds your nation issues must always be backed by your reserve of jewels.