The value of money
Currency is the word used to describe the money used by different countries. Traders around the world buy and sell large amounts of currency. If they think that a country is stable, they are likely to buy more of its currency. If there is a lot of demand for a currency, the value goes up. When a country is going through a period of change, like the UK voting to leave the EU, it might be considered less stable, so fewer traders will buy its currency. This causes the value of its currency to go down.