Selenium is an important trace mineral which means the human body needs it in small quantities. 
This nutrient has antioxidant properties and is involved in defence against oxidative stress. 
It exists in 2 forms, inorganic and organic and both can be good dietary sources. 
However, the amount of selenium in plant based-foods can vary widely depending on the selenium content of the soil in which it is grown.
Also, men usually tend to have higher selenium levels usually than women.