They are subject to fixed foreign currency rates that are usually set by the central bank of a country. For example, the Hong Kong dollar is fixed and does not change unless the central bank changes the rate.
Other currencies fall under the category of being a partially floated currency. Here, the foreign currency rates for that currency are allowed to change within the limits set by the government. They do this so that the foreign currency rates of their currency move less and are more stable.
The introduction of the Euro created a single European currency for most of the European countries and eliminated all foreign currency rates in those countries. A hamburger may cost a different price in Spain and Germany, but they are both paid for in Euros.
If a country is experiencing political or economic problems, you may see their foreign currency rates change very frequently as buyers and sellers try to adjust to the changing conditions.
Some people make money trade the changes in the foreign currency rates. They buy one currency with their dollars, hoping that the foreign currency rates will move in their favor so that later when they buy back their dollars, they will get more dollars than they originally started with.
Foreign currency rates are a fact of business life in the modern world. They determine the price of all our important goods and the cost of our overseas vacations. Foreign currency rates also have a bearing on the price we pay for gasoline and other basic commodities.