This means that the chemical is in the form of a levorotatory isomer. For a chemist or physicist, there is no particular difference between levorotatory and dextrorotatory or racemate (a mixture of enantiomers), but for receptors and enzymes in the body, it means a lot. Most drugs are in the form of racemates and usually only one of them works. In some drugs, only one isomer is present, for example, L-dopa, L-thyroxine, etc. More details can be found at https://ru.m.wikipedia.org/wiki/%D0%AD%D0%BD%D0%B0%D0%BD%D1%82%D0%B8%D0%BE%D0%BC % D0% B5% D1% 80% D1% 8B
If I understand you correctly, then this is something like L-thyroxine, L-arginine. L is the designation of the levogyrate optical isomer (enantiomer) of a substance. Some substances have the same chemical composition, but they can be in different isoform-states. In this case, different optical isomers are essentially mirror images of each other. They can have completely different properties and effects on the body: D-metorphan (dextromethorphan) is a dissociative, and L-metorphan (levomethorphan) is an opioid analgesic. Isomers may have different affinities for the same receptors, so an isolated isomer may be more effective than a mixture (racemate) from which this isomer has not been isolated.