Amortization is the process of paying off a loan gradually through regular payments. Each payment is split between interest and principal, but the mix shifts over the life of the loan: early payments are mostly interest, and later ones are mostly principal. An amortization schedule lays out that split payment by payment.
The principal portion of every payment is what builds your equity — sometimes called the “loan paydown” return of owning a rental. A standard 30-year fixed loan fully amortizes to a zero balance by the end of its term; some loans instead are interest-only or end in a balloon payment, meaning they amortize slowly or not at all. (The same word is also used in accounting for spreading the cost of an intangible asset over time.)