I need explanation quickly.

Every atom has some characteristic mass of its own and this is known as its atomic mass. All the atoms of an element have the same atomic mass. Since it is difficult to determine the mass of an individual atom, so the mass of an atom is ascertained in relation to the mass of C-12 isotope. Thus, atomic mass is in fact relative atomic mass. C-12 is an isotope of carbon and its mass is used as a standard reference to calculate the relative atomic masses of all elements. The IUPAC adopted one-twelfth of the mass of a C-12 atom as the standard unit to measure relative atomic masses. It named this unit as atomic mass unit (amu) or unified atomic mass unit (u).

So,