The Balance of Payments

A country's balance of payment account records all transactions between the residents of that country and the rest of the world, These transactions enter as either debit items or credit items. The debit items include all payments to other countries: these includes the country's purchases of imports, the spending on investment it makes abroad and the interest and the interest and dividends paid to foreigners who have invested in the country.  
The credit items include all receipts from other countries: from the sales of exports, from investment expenditure by foreigners in the country and interest and dividends earned from abroad.

The sale of exports and any other receipts earn foreign currency. The purchase of imports or any other payments abroad use up foreign currency. If we start to send more foreign currency than we earn, one of two things must happen. Both are likely to be a problem

The balance of payments will go into deficit. In other words, these will be a shortfall of foreign currencies.
The government will therefore have to borrow money from abroad, or draw on its foreign currency reserve to make up the shortfall. This is a problem because, if it goes on too long, overseas debts will mount, along with the interest that must be paid; and/or reserves will begin to run low.

The exchange rate will fall. The exchange rate is the rate at which one currency exchange for another. For example, the exchange rate of the pound into the dollar might be 1pound = 1.20 dollar