What is reducing agent?How does it work?

A reducing agent (also called a reductant or reducer) is the element or a compound in a redox (reduction-oxidation) reaction that reduces another species. In doing so, it becomes oxidized, and is therefore the electron donor in the redox. For example consider the following reaction:

2Mg(s) + O2 → 2Mg2+(s) + 2O2-
The reducing agent in this reaction is magnesium. Magnesium donated its two valence electrons, has become an ion, and allows itself as well as oxygen to become stable.
Reducing agents need to be protected from air because they react with oxygen.