what is currency trading?

The foreign exchange market - Forex, FX, or currency market- is a global decentralized or Over-The-Counter -OTC- market for the trading of currencies.

The FX market determines the foreign exchange rate - buying, selling and exchanging currencies at current or determined prices.

https://www.thebalance.com/what-is-currency-trading-1344894

http://www.forextips.com/forex-101/what-is-currency-trading/

Just like the name implies its trading money.
Such as you buy pesos hoping they go up in value against other currencies.
That's an oversimplified explanation but its the basics

The trading of currency.

trading in the world's different currencies

Currency (MONEY) can be traded on some international markets as a commodity, just like precious metals, corn or oil. If you're going to gamble in the commodities market, it's best to know what you're doing BEFORE you start.