I answered this somewhere else. Arginine is an amino acid that is supposed to be a precursor to Androgens and also supposed to bring blood flow to the area, resulting in better penile and clitoral engorgement, thus better feeling and maybe stronger orgasms.
Some people are quite sensitive to it and avoid it. Anyone with any form of HSV (herpes, cold sores, anything in the herpes family) can be triggered by using L-Arginine. These people need to stay away from it.