In the context of housing loan, it is simply the process of paying off a loan balance. As you make payments, a certain amount goes toward paying the principal and certain amount to interest. The schedule or table of amortization shows the declining balance as you make payment.
To add on, negative amortization is a type of loan situation that occurs when the monthly payment is not enough to cover the principal or interest, Instead of a declining balance, the balance on the loan actually increases.