correlation of 1 means it is completely the same and -1 the complete opposite. You need it for basket trading. You take pairs of e.g. USD like EURUSD, GBPUSD, AUDUSD and GOLD with an high correlation and when one pair moves down or up, one will follow and the others must do the same with some delay. Look for reversal patterns or breakouts. Some beginners in Basket Trading use a Currency Strength Meter. Correlation is cooler and actually works.