Question

What is meant by a.m.u?

Answer 1

It stands for "atomic mass unit", it is a unit used by scientists to measure the mass of atoms and molecules.
The amu is defined in a way such that one atom of carbon-12 will weigh exactly 12 amu. We initially used oxygen, but there were a few different oxygen-based definitions around (e.g. physicists were using a specific isotope of oxygen, chemists weren't, etc, so this was confusing). Carbon was chosen as a new standard that everyone agreed on.