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

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.