Good one
Nitrogen is a compound that is unique to protein and can provide information about someones amino acid status (amino acids are found in proteins,there are 20 essential and non essential combined,essential are provided by food,non essential are produced by the liver e.g Glutamine)
All macro's contain hydrogen,carbon and oxygen but protein is the only macro that contains the additional nitrogen molecule,you can measure excretion of nitrogen as to how good the body is at muscle building.
If your nitrogen excretion is high this should make you increase your protein intake to make up the difference and build muscle.There are 3 states of nitrogen balance
Positive
Equilibrium
Negative
With positiving being optimal for muscle growth and negative being bad.
I hope that was relatively easy to understand if not i'll try and rephrase it