Correlation between 2 signals means you can say something about one of them by observing the other.

If you mean the standard correlation, $ E[xy] $, it means you knowledge second moment statistics.

Correlation between 2 signals means you can say something about one of them by observing the other.

If you mean the standard correlation, $ E \left[ x y \right] $, it means you knowledge second moment statistics.
Which implies one can, using only linear functions, estimate one from the other to some level.