*This is a very general overview
Selenium is a trace element or nutrient that we need for overall good nutrition but it is only needed in tiny amounts. It is required in making selenoproteins which are antioxidant enzymes. Antioxidant enzymes are known to have properties that protect cells from free radicals.
Current research (2009) on selenium in not conclusive but it has suggested that a higher/normal level of selenium intake can help prevent some cancers. Selenium is also important in thyroid regulation and helping the immune system function. However in one study, elevated intakes of selenium was linking to a greater risk in developing type II diabetes.
It is important to note that selenium deficiencies in the USA are very rare. The normal US diet should provide enough selenium and supplements are not usually needed except in the rare cases of certain disease.