An enantiomer is a pair of structures which are mirror images of each other. For D-glucose:
CHO | CHO
| | |
H--OH | OH--H
| | |
OH--H | H--OH
| | |
H--OH | OH--H
| | |
H--OH | OH--H
| | |
CH2OH | CH2OH
On the left is D-glucose, and on the right is L-glucose, mirror images.