L-Arginine is a conditionally essential amino acid found in the diet. It is a dietary supplement used mostly by athletic people because it is the amino acid that directly produces nitric oxide via the nitric oxide synthase enzymes.
It's particularly important during periods of illness and chronic conditions like hypertension and type II diabetes, as these states tend to be characterized by an increase in the enzyme that degrades L-arginine (known as arginase) resulting in a transient deficiency; this precedes an increase in blood pressure in these states, and can be partially remedied by an increase in L-arginine intake or resolution of the illness/disease state.
L-arginine is a popular supplement for athletes as it is touted to increase nitric oxide activity in the body. Unfortunately, this effect appears to be unreliable in otherwise healthy adults. While there have been studies that have measured increased effects of nitric oxide (blood flow) they are paired with studies showing no net effects. As for the mechanism, there is reason to believe that this difference is in part due to poor absorption of L-arginine from the intestines before it can reach appreciable activity in the body.
L-Citrulline is another supplementation option because it is converted into arginine in the kidneys. It also has a better absorption rate. Citrulline is able to increase levels of plasma arginine more effective than arginine itself. For long-term health-related supplementation, L-citrulline may be a better supplementation option than L-arginine.