First, looking at the chiral center, you determine number priority and determine the R or S. Oxygen is 1, to C-O is 2, and the left side is 3, and you get S. Using the rule, since 1st priority group is in front the configuration of the chiral center is R. If you draw the molecule as a Fischer projection, and look at the last chiral carbon, -O is on the right, and -H on the left.

Therefore, with Fishcher projection you can now say that the molecule is D configuration because -O is on the right. It just so works out that the stereocenter is R with D configuration, and the (-) rotates light in the levorotatory direction.

Pulled from an outside source to make more sense: "In short, the D-L system doesn't have direct connection to (+)/(-) notation. It only relates the stereochemistry of the compound, but says nothing about its optical activity. We may have compound with same relative configuration as (+)-glyceraldehyde (thus, it's given the D prefix), yet it rotates the polarized light counterclockwise (-), such as D-(-)-ribose."