That's the covariance of X and Y.
It tells you, in a way, "how much" X and Y are correlated, with covariance 0 meaning uncorrelated (Not to be confused with independent).
Because $\langle A\rangle$ and $\langle B\rangle$ can be quite large, it is customary to define the correlation coefficient: ($cov(A,B)$ is the covariance of A and B)
Which is then bounded between -1 and 1, and tells you how correlated they are. Either positively correlated (Positive values far from 0), not correlated (Values close to 0), or negatively correlated (Negative values far from 0).
I've yet to see any physicist use this, at least in theoretical calculations.
It can be useful in data analysis, trying to found out if two parameters are correlated, but other than that, I can't think of other uses in physics.