Forex is a commonly used abbreviation for "foreign exchange," and it is typically used to describe trading in the foreign exchange market by investors and speculators.
For example, imagine a situation where the U.S. dollar is expected to weaken in value relative to the euro. A forex trader in this situation will sell dollars and buy euros. If the euro strengthens, the purchasing power to buy dollars has now increased. The trader can now buy back more dollars than they had to begin with, making a profit.
This is similar to stock trading. A stock trader will buy a stock if they think its price will rise in the future and sell a stock if they think its price will fall in the future. Similarly, a forex trader will buy a currency pair if they expect its exchange rate will rise in the future and sell a currency pair if they expect its exchange rate will fall in the future.