A depreciation is a fall in the external value of one currency against another, for example the Australian dollar might depreciate against the US dollar so that one Australian dollar buys less of the US currency.
Here are some reasons why the exchange rate of a country might depreciate:
Countries running large / persistent current account deficits on their balance of payments tend to see weak currencies. Likewise, a currency can falling in value if the macroeconomic fundamentals worsen i.e. there is slower growth, falling profits for domestic and overseas investors and when the markets feel that the real exchange rate is over-valued and due for a correction.
Japanese Yen against the US dollar