Retail Sales measure the change in the total value of sales at the retail level. It is the foremost indicator of consumer spending, which accounts for the majority of overall economic activity.
A higher than expected reading should be taken as positive/bullish for the USD, while a lower than expected reading should be taken as negative/bearish for the USD.
Why Traders Care
Consumer spending accounts for more than two-thirds of the economy, so if you know how the consumer sector is faring, you'll have a pretty good handle on where the economy is headed. Needless to say, that's a big advantage for traders.