What exactly is selenium? Why should the human body supplement selenium?

Selenium is an essential trace element for human body! It exists in soil or rock! In fact, the normal human body needs nothing to supplement! Selenium supplement products are gimmicks! Use people's lack of health knowledge and medical common sense to promote products! The so-called differentiated marketing means! Healthy human trace elements are normal! If you supplement too much, the balance of the human body will be broken, but it is not good!