What does it mean when I see the letter L- or D- printed in front of the molecule name on a supplement bottle label (e.g., L-arginine or D-fructose)?
Chemists use L- and D- distinction to differentiate between the two different forms of a chiral compound molecule. Called optical 
isomers or enantiomers, the forms are chemically identical but structurally different. By convention, these are called L- and D- forms, analogous to left-handed and right-handed configurations (based on Latin words, laevus and dexter for left and right). In both forms of the molecule, atomic makeup is exactly the same in composition. That is, they both have the same identity, number and order/sequence of constituent atoms. The atoms, however, are arranged differently in space. In the most simple terms, optical isomers appear as mirror images of each other.
You might think of the situation like a pair of hands or a pair of gloves. The two of them seem to be alike. They have the same 
size, each with exactly four fingers and a single thumb. Because of the way they are flipped around (oriented differently in space), though, there is no way you could ever rotate a right hand to make it fit exactly like a left one, or vice versa. No matter how the two hands are oriented, twisted or moved, it is impossible for all the major features of both hands to coincide precisely. Similarly, for a chemical compound, the carbon atom with four different attachments has two possible unique configurations. They, too, assume separate spatial arrangements that are not identical, just like the right/left hands in the hand-glove example.
What Difference Does It Make?
Because of the difference in form, necessarily the two isomers interact differently with receptors in the body. Going back to the hand-glove analogy, shaking someone’s right hand is much less awkward to do with your own right hand compared to your left. So, too, the different isomers also interact more or less easily depending on the placement of specific receptors. In the case of amino acids, for example, human cells manufacture only L-amino acids to incorporate into proteins. While amino acids can occur in both L- and D-forms, only the L-forms of the molecule are actually used by cells in the human body.
What Does It Mean If the Name Includes DL- ?
Occasionally you might see a label with DL- noted at the beginning of the supplement name, for example, DL-phenylalanine. This notation means the bottle contains 50% by weight the D- form of the molecule and 50% the L- form.