What is it; and how does it help atthletes?

If you mean Glutathione then chemically it reduces oxidative stress- I don't know anything about taking it as a supplement though - not my area. It is present in all cells.

Oxidative stress is caused by free radicals being produced by the mitochondrion during oxidative phosophorylation (making energy for the cell)- this would increase during exercise. Therefore a supplement which reduces this could prevent cell damage.

If you want a cheaper way to reduce free radicals in your body - eat more fruit!

That's the science as for practice I don't know

Glutathione (L-gammaglutamyl-L-cysteinylglycine) is a tri-peptide of the amino acids cysteine, glycine, and glutamic acid. Glutathione is an antioxidant compound found in living animal and plant tissue. It takes up and gives off hydrogen and is important in cellular respiration. A deficiency of glutathione can cause hemolysis (destruction of red blood cells, leading to anemia) and oxidative stress. Glutathione is essential in intermediary metabolism as a donor of sulfhydryl groups which are essential for the detoxification of acetaminophen. [PDR Medical Dictionary. Spraycar. 1999] Selenium is a structural component of, and a co-factor for the antioxidant enzyme glutathione peroxidase.

Glutathione is the major endogenous antioxidant produced by the cell. Glutathione participates directly in the neutralization of free radicals, reactive oxygen compounds, and maintains exogenous antioxidants such as vitamins C and E in their reduced (active) forms. In addition, through direct conjugation, glutathione plays a role in the detoxification of many xenobiotics (foreign compounds) both organic and inorganic. Glutathione is an essential component of the human immune response. Proposed mechanisms of immune enhancement include:
Optimizing macrophage functions,
Offsetting oxidative damage associated with lymphocyte monoclonal expansion, and
Stabilizing the mitochondrial membrane thereby, reducing apoptosis in lymphocytes

Source: Nutrition Advisor: Your source for Glutathione | Calorad | Calorad 2000 | Whey | Immunocal | Immunocal Platinum | Aroma Patch | December 28, 2006