Reduction is a chemical reaction that involves the gaining of electrons by one of the atoms involved in the reaction between two elements. The term refers to the element that accepts electrons, as the oxidation state of the element that gains electrons is lowered.
An example of a reduction is when iron reacts with oxygen, forming a chemical called rust. In that example, the iron is oxidized and the oxygen is reduced. This is called redox. A blast furnace reverses that reaction, using carbon monoxide as a reducing agent to reduce the iron.