What is an atomic mass unit? why is it necessary to introduce such a unit?

Explanation:

Atomic mass unit is defined as 1/12 of mass of the carbon-12. It is represented by amu. It is a relative mass unit.

The masses of atoms are very very small.  For example, atom of O-16 has the mass of about 2.66×10⁻²³ g. Thus, such small numbers corresponds to many unnecessary calculations and difficulty to remember and to deal with them. Thus, scientists decided on using carbon-12 nuclide as a reference standard by means of which all the other masses are compared compared.  

For example, The mass of one atom of helium-4 is 4.0026 amu.

every chemical equation adheres to the law of conservation of mass, which states that matter cannot be created or destroyed. therefore, there must be the same number of atoms of each element on each side of a chemical equation.