Correlation is a statistical measure that tells you how much two things move together.
Correlation values range from -1 to +1. A correlation of +1 indicates a perfect positive relationship, meaning that every time one variable increases, the other increases proportionately.
A correlation of -1 implies the reverse—a perfect negative relationship. Every time one variable increases, the other decreases proportionately.
A correlation of zero (0) means there is no linear relationship between the variables.
It's important to remember that correlation does not imply causation. It simply identifies the strength and direction of a relationship, not the underlying cause of it.
Meaningful correlations are typically 0.40 or above. Strong correlations are 0.60 or above. (Source: "Statistical Methods for the Social Sciences" by Alan Agresti and Barbara Finlay)