In the name α-D Glucose,what do "α" and "D" mean?

"α" and "D" refer to "the configurations of different carbon atoms in the molecule.

In a sugar, the D or L designation refers to the configuration of the chiral carbon farthest from the aldehyde or keto group. This is C-5 in glucose.

In a Fischer projection, the highest numbered chiral carbon has the OH group pointing to the right.

D- and L-sugars are mirror images of one another. They have the same name, for example, D-glucose and L-glucose.

The carbon that determines the D and L designations (C-5 in glucose) is also the one that determines the α and β configurations.

The terms α and β refer to the configurations of the anomeric carbons in the "usual" Haworth projections (C-1 for glucose).

The α-anomer is the isomer with the OH group on the side opposite to the C-5 substituent (the CH₂OH group), i.e. pointing "down".

The β-anomer has the anomeric OH group on the same side as the CH₂OH group, i.e. pointing "up".