L-Arginine is naturally occurring semi-essential amino acid that converts in the body into a chemical called nitric oxide. Nitric oxide causes the blood vessels to open wider for improved blood flow (and as such can help improve the erection quality).
Clinical trials have shown little evidence of the effectiveness of using L-Arginine for treating Peyronie's disease as such. There are however some studies indicating that L-Arginine may benefit some men with erectile dysfunction. Many men with Peyronie's also suffer from erectile issues and L-Arginine may be considered worth trying in those cases.
For more information about the pros and cons of using L-Arginine to treat Peyronie's disease.