Like most assets, a currency is ruled by supply and demand. When the demand for something goes up, so does the price. If most people convert their currencies into yen, the price of yen goes up, and yen becomes a strong currency. Because more dollars are needed to buy the same amount of yen, the dollar becomes a weak currency.
Currency is, after all, a type of commodity. For example, when a person exchanges dollars for yen, they are selling their dollars and buying yen. Because a currency’s value often fluctuates, a weak currency means more or fewer items may be bought at any given time. When an investor needs $100 for purchasing a gold coin one day and $110 for purchasing the same coin the next day, the dollar is a weakening currency.