Ryan is right, selenium is an antioxidiant! If a horse miss selenium, the repercussions can be a muscle degeneration, the fragility of red blood cells, muscle aches... If you give him too much selenium, he can have an intoxication or
serious health problems (deformation and loss of the hoof, cardiorespiratory issues, loss his eyesight...)!
From what I've seen, Selenium is an antioxidant, a quite powerful one.. It helps oxygenate muscles and get rid of lactic acid, making it rather important for the health of our horses. When they do lack selenium, they can experience muscles soreness and be less flexible, especially in the hips and back, similar to a tying up.. Hope it helps a bit! :)