Question

What are Redox reactions???

Answer 1

Redox is a chemical reaction in which the oxidation states of atoms are changed. Any such reaction involves both a reduction process and a complementary oxidation process, two key concepts involved with electron transfer processes.

Answer 2

Here is the answer

Redox Reaction​ 

In this reaction, An oxygen atom goes away from CuO { Copper oxide } , which means that reduction of copper oxide takes place, whereas hydrogen molecule takes up oxygen atom and water { H2O } is formed , hence, Oxidation of hydrogen takes place.
{ CuO + H2 --> Cu + H2O }
This is the reaction.

Thus Oxidation and Reduction reactions occur simultaneously . Due to this characteristic of the reduction and oxidation reaction, a single term " Redox " reaction is used in place of two terms.

In fact, 

Redox = Reduction + Oxidation.

For Example,

MnO2 + 4 HCl ---> MnCl2 + 2 H2O + Cl2.

2 H2S + SO2 ---> 3S + 2 H2O.