Most brokers also provide leverage. Many brokers in the U.S. provide leverage up to 50:1. Let's assume our trader uses 10:1 leverage on this transaction. If using 10:1 leverage the trader is not required to have $5,000 in their account, even though they are trading $5,000 worth of currency. They only need $500. As long as they have $500 and 10:1 leverage they can trade $5,000 worth of currency. If they utilize 20:1 leverage, they only need $250 in their account (because $250 * 20 = $5,000).
The foreign exchange market is where currencies are traded. Currencies are important to most people around the world, whether they realize it or not, because currencies need to be exchanged in order to conduct foreign trade and business. If you are living in the U.S. and want to buy cheese from France, either you or the company that you buy the cheese from has to pay the French for the cheese in euros (EUR). This means that the U.S. importer would have to exchange the equivalent value of U.S. dollars (USD) into euros. The same goes for traveling. A French tourist in Egypt can't pay in euros to see the pyramids because it's not the locally accepted currency. As such, the tourist has to exchange the euros for the local currency, in this case the Egyptian pound, at the current exchange rate.