It is a food preservative found in processed meats, and used particularly in curing meat such as Italian salamis. It preserves the colour, keeping meat pink or red. More importantly, though, it helps to inhibit the development of botulism.
Sodium Nitrate occurs naturally in leafy green vegetables.
Towards the back of your tongue, there are actually microbes whose job is to change nitrate into nitrite, to help prevent bacterial poisoning. In your stomach, the nitrite gets broken down, releasing nitric oxide, which helps to kill off Salmonella and Escherichia coli. Most Sodium nitrate leaves your body in 5 hours through urine.
“It has been reported that people normally consume more nitrates from their vegetable intake than from the cured meat products they eat. Spinach, beets, radishes, celery, and cabbages are among the vegetables that generally contain very high concentrations of nitrates (J. Food Sci., 52:1632). The nitrate content of vegetables is affected by maturity, soil conditions, fertilizer, variety, etc. It has been estimated that 10 percent of the human exposure to nitrite in the digestive tract comes from cured meats and 90 percent comes from vegetables and other sources.” 
The chemical formula for Sodium Nitrate is NaNO3.
 Richard J. Epley, Paul B. Addis and Joseph J. Warthesen. Nitrite in Meat. University of Minnesota Extension office. WW-00974. 1992. Retrieved September 2010 from http://www.extension.umn.edu/distribution/nutrition/DJ0974.html