Hi Madan,

AMU is the atomic mass unit and this is a physical constant which is equal to 1/12 the mass of an unbound carbon atom. It is a unit of mass which is used to express the units of the quantities such as molecular mass and atomic mass. It is used in the SI measurement system. Carbon atom has 6 protons and 6 neutrons in its nucleus and in short it is the average rest mass of proton and neutron.

I hope you got the answer.

All the best !