This is the 11339th most frequent Russian word.
десятой
The word "десятой" is the feminine or instrumental form of "десятый," meaning "tenth" in English.
In this sentence, the word "десятой" is used as an ordinal numeral modifying "странице" (page), indicating its sequential order.
На десятой странице книги найдётся ответ.
On the tenth page of the book, there is an answer.
Here, "десятой" is used as a fractional numeral to indicate a part (a tenth) of the cake.
На десерте осталась лишь десятой торта.
Only a tenth of the cake was left on the dessert.
In this example, "десятой" modifies "темы" (topic), specifying that a particular numbered topic is being discussed.