What is Selenium?
“Selenium (Se) is a mineral that exists in poultry, fish, cereal, and eggs. It was considered a toxin until 1957; however, current research shows eating proper amounts of Selenium has many apparent health benefits.
Plants take in Selenium from soil; thus, people around the world eat different daily amounts of Selenium depending of the concentration of the mineral in the surrounding geography.
Therefore, depending on geography, individuals may suffer from Selenium deficiency. Current scientific research is looking at redefining the recommended values of Selenium.” 
Why is selenium necessary?
“Selenium has antioxidant properties that help the body prevent cellular damage from free radicals, and one of its most valuable roles is as a cofactor of an important antioxidant enzyme in the body called glutathione peroxidase. Selenium also helps support a strong immune system, regulates thyroid function, and may help reduce the risk of prostate and secondary cancers. It also plays a role in the prevention of cataracts and heart disease.” 
Sources of Selenium
“Seafoods and organ meats are the richest food sources of selenium. Other sources include muscle meats, cereals and other grains, and dairy products. The amount of selenium in drinking water is not nutritionally significant in most geographic regions [2,6]. The major food sources of selenium in the American diet are breads, grains, meat, poultry, fish, and eggs.
The amount of selenium in a given type of plant-based food depends on the amount of selenium in the soil and several other factors, such as soil pH, amount of organic matter in the soil, and whether the selenium is in a form that is amenable to plant uptake [2,6,8,9]. As a result, selenium concentrations in plant-based foods vary widely by geographic location. For example, according to the U.S. Department of Agriculture Food Composition Database, Brazil nuts have 544 mcg selenium/ounce, but values from other analyses vary widely.
The selenium content of soil affects the amounts of selenium in the plants that animals eat, so the quantities of selenium in animal products also vary. However, selenium concentration in soil has a smaller effect on selenium levels in animal products than in plant-based foods because animals maintain predictable tissue concentrations of selenium through homeostatic mechanisms. Furthermore, formulated livestock feeds generally contain the same levels of selenium.” 
 Self Hacked - Top 10 Science-Based Health Benefits of Selenium
 Dr Weil MD - Selenium
 National Institute of Health - Selenium