You can think of currency devaluation as a kind of nationwide sale. There are thousands of businesses in China that sell goods and services to customers in foreign countries like the United States. Their goods are generally priced in China's own currency, the yuan. So if the yuan becomes less valuable relative to the dollar, Chinese imports suddenly become cheaper here in America. In other words, when the yuan falls by 4 percent, as it has over the past few days, it's as if every business in China cut its prices for Americans by 4 percent.
And just as sales help stores sell more of their products, a currency devaluation helps countries sell more exports, boosting the economy. Right now the Chinese economy is in the midst of an economic slowdown and has suffered from stock market turmoil, so it can use some extra help.