Your everyday accounting may likely include balances that you’re essentially required to settle in some foreign currency (that is different currency than you’re using in your everyday accounting).
Monetary balances, that is balances that require to be settle in cash (i.e. you need to pay for them in cash or you shall be paid in cash) should be revalued at each balance sheet date using the exchange rate that is effective at that date. For an example say that you have a payable balance in 100 FCU (foreign currency unit) and the exchange rate as at 30.11.2014 when you recognized the payable was 1 FCU = 1.5 CU (currency unit your accounting is daily done in). As at 31.12.2014 the exchange rate has change and it’s now 1 FCU = 1.2 CU. When initially your payable was effectively 150 CU (100 x 1.5), as at 31.12.2014 (the next balance sheet date) it’s 120 CU (100 x 1.2). The change in the payable (150 less 120) is recognized as a part of your non-operating income (that is it’s not sales revenue).
Note here that the exchange rates to use for revaluing your balances should always first off be the same and it should normally be either a central bank or something similar. It should not be your local currency exchange office or kiosk.