How does it work?

Selenium is a trace mineral found in soil, water, and some foods. It is an essential element in several metabolic pathways. There is some evidence that low selenium levels are associated with an increased risk of prostate cancer. In human studies, initial evidence has suggested that selenium supplementation reduces the risk of developing prostate cancer in men with normal baseline PSA (prostate specific antigen) levels, and low selenium blood levels.

Dangers and possible side effects....

Avoid if allergic or sensitive to products containing selenium. Avoid with a history of nonmelanoma skin cancer.

References:

http://www.webmd.com/a-to-z-guides/supplement-guide-selenium

http://www.healthline.com/health/selenium-an-essential-mineral