Correlation implies that as one variable changes, the other also changes.
Sometimes, though, if we know that there is good correlation between two characteristics, we can use one to predict the other, particularly if one characteristic is easy to measure and the other isn't. For instance, if we prove that weight gain in the first trimester of pregnancy correlates well with fetus development, we can use weight gain as a predictor. The alternative would be expensive tests to monitor the actual development of the fetus.