What are NormalizedDebt and NormalizedDebt?

Hi, I am trying to understand how Aave smart contracts work. I cannot understand many of the variables in the ReserveLogic contract. For instance, what is NormalizedDebt? Why do you find the amount of the debt of a user by multiplying NormalizedDebt by the number of DebtToken the user has? I mean how come all the users no matter when they have started borrowing will have the same NormalizedDebt per debt token?