This is the 17805th most frequent Russian word.
уплаченной
"Уплаченной" means "paid" in English.
In this sentence, 'уплаченной' is used as a past passive participle agreeing with 'sums', describing that this portion of money has been paid.
Согласно условиям договора, часть уплаченной суммы не подлежит возврату.
According to the terms of the contract, part of the paid amount is non-refundable.
Here, 'уплаченной' describes 'книги' and conveys the act of payment already made by the subject.
Они вернули мне стоимость уплаченной мною книги.
They refunded me the cost of the book I paid for.
In this sentence, 'уплаченной' modifies 'налоговой сумме', indicating that the taxes have already been paid beforehand.