Nitric oxide comes up constantly in circulation supplements, but what it actually does is often skipped over. Here's the short version.
Nitric oxide is a gas your body produces naturally, acting as a signal that tells the smooth muscle lining your blood vessels to relax. When that muscle relaxes, the vessel widens, and blood can move through with less resistance.
L-Arginine is the direct raw material your body uses to make nitric oxide. More available arginine generally means more capacity for nitric oxide production, assuming the rest of the process is working normally.
Blood vessels that relax more easily allow blood to flow with less effort, which is part of why nitric oxide support is such a common thread in blood pressure and circulation-focused supplements.