Reduction is the gain of one or more electrons by an atom during a chemical reaction. It is always part of a redox reaction. In living organisms, chemical energy is stored in high-energy electrons that are transferred from one atom to another in reactions involving oxidation and reduction.
A reducing agent is a substance that causes reduction reactions. Thus in the reduction of oxygen to form water, 2H2 + O2 → 2H2O, hydrogen is the reducing agent. In biological systems, some sugars act as reducing agents.