Strong, negative correlation.
So with correlations, they have two categories that they fall under.
Firstly, they either fall under positive or negative correlation. It's as it seems: positive correlation coefficients are positive numbers, negative correlation coefficients are negative numbers. In this case, our correlation is negative.
Next, they either fall under weak or strong correlation. If the absolute value of the correlation coefficient is closer to zero, then it's considered a weak correlation. However, if the absolute value of the correlation coefficient is closer to one, then it's considered a strong correlation. In this case, the absolute value of -0.91 is 0.91, and since that is closer to 1 this means our correlation is strong.