What is D and L?

A sugar or an amino acid is drawn in Fischer projection with its most oxidised carbon at the top. It is D if the $\ce{OH}$ on the stereocentre farthest from the top (or, for an $\alpha$-amino acid, the $\ce{NH2}$ on the $\alpha$ carbon) points to the right, and L if it points to the left. The descriptors compare configurations with that of glyceraldehyde; they are not the CIP descriptors.