What are reduction reactions? Give examples.

Reduction refers to the loss of oxygen or gain of hydrogen by the substances and the reactions are known as the reduction reactions.
For example,
When hydrogen gas is passed over the heated CuO, the black coating on the copper again turned brown because the reverse of oxidation has occurred over here
$\mathrm{CuO}+H_2\to \mathrm{Cu}+H_{2}O$.