What do D and L (like R and S) indicate in enantiomers?
The D and L prefixes refer to the side of the highest-numbered chiral centre in a Fischer projection.
The prefix "D" comes from the Latin word dexter. It means "on the right side".
The prefix "L" comes from the Latin word laevo. It means "on the left side".
We usually see the terms D and L when talking about sugars and amino acids.
For example, glucose exists as D-glucose and L-glucose. Here, the D and L refer to the Fischer projections.
If the OH in the highest-numbered chiral carbon (closest to the bottom) is on the right, we have a D-sugar. If the OH is on the left, we have an L-sugar.
In amino acids, the D and L refer to the location of the NH₂ group in the Fischer projection.
The above diagram shows the structure of L-alanine (NH₂ on the left) and D-valine (NH₂ on the right).