The chemical structure of compounds can be written out in a two dimensional drawing on paper. In reality, compound have a three dimensional shape. Think of a molecule being left-handed or right-handed. Each is a stereoisomer, like a mirror image of the other. An enantiomer is one of two stereoisomers. The structures can not be superimposed on one another. The prefix lev- is often attached to the R-enantiomer. The two enantiomers have different chemical properties because the molecules they interact with are enantiomers themselves. We usually prescribe medications that are racemic, that is, they are an equal mixture of both enantiomers. Recently the U.S. government recognized enantiomers as separate medical compounds and granted them patents. The pharmaceutical industry has seized this as a way to generate more profits from compounds are discovered and marketed with less adverse effects.
Remember the racemic compounds literally do contain the enantiomers listed. Did you know they were nearly the same compound? What happens to the L-enantiomer? Usually this is responsible for some of the adverse effects and not much of the beneficial effects found in the racemic compound.