Arginine or L-Arginine is a conditionally essential amino acid found in the diet. It is mostly used by athletic people and people who undertake intense gym training 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 in chronic conditions like hypertension and type II diabetes, as these states tend to be characterized by an increase in the enzyme that degrades arginine resulting in a transient deficiency; this precedes an increase in blood pressure in these states, and can be partially remedied by an increase in arginine intake or resolution of the illness/disease state.
The effect of arginine is 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.