Relative Atomic mass concept

 A concept that looks too hard is easily simplified here :

How do we measure the mass of the subatomic particles , or masses of atoms 

We can not compare them to the Kg or even the gram since they are very very tiny , otherwise the mass will show us a very small funny number

so we have to compare their masses to a smaller fixed unit which is reasonably small that their masses can compare to

we will take the carbon atom as our reference unit , and we will say that the carbon atom has 12 units . so each unit or *relative atomic mass unit = 1/12 of mass of carbon atom

Anything that we measure after that should follow this unit

like 

A proton has a mass unit of 1 , that is 1/12 of the mass of Our reference carbon atom

A Neutron also has the the same mass unit of 1 = 1/12 of the carbon atom

The carbon atom that we use has to be of a special type , it should be from the type that contains 6 protons and 6 neutrons , so its total mass is 12 units , we call this the carbon 12 atom 

Are there any other carbon atoms , 

yes the carbon 14 is an isotope of carbon 12. It has 6 protons but different number of neutrons (6) . we don't use this isotope as reference , we use only carbon 12 atom as our reference .