The currency of different countries is constantly changing. You see in the newspaper that sometimes the dollar is going strong against the money, sometimes the money is strong against the dollar.
Most of the world's currency is the rate. So, if you are buying a dollar, the price of the euro against the dollar goes down, then you can sell the dollar and buy the euro. Again, if the euro is stronger against the dollar, you can earn more dollars by selling the euro.
Maybe you had $ 5 that you sold for 5 euros. Later, after selling the euro, it sold for $ 120. That way you can earn. We can profit only if the share price is increased in the stock market. But in the Forex Market, no matter whether the currency is strong or weak, we have the opportunity to profit, which is the biggest advantage of the Forex market.