The letter 'D' in D - glucose signifies :

D & L represent relative configuration. Reference Compound is Glyceradehyde.

D, L notation does not have any relation with specific rotation in Carbohydrate Chemistry:
If the configuration at the Penultimate Chiral Carbon (or) highest stereogenic centre resembles D–glyceraldehyde, the carbohydrate is assigned D–configuration. 

structure of glucose is

Highest stereo genic centre (or) Penultimate chiral carbon is $5^{\mathrm{th}}$ carbon. 
Configuration at $5^{\mathrm{th}}$ carbon resembles D–glyceraldehyde.