This is the 782nd most frequent Czech word.
ti
The Czech word "ti" means "you" (dative case, plural or formal singular).
Here, 'ti' is a demonstrative pronoun translating to 'those', used to refer specifically to a certain group of people.
Ti lidé jsou velmi přátelští.
Those people are very friendly.
Here, 'ti' is the dative form of the second person singular pronoun, meaning 'to you', indicating the person receiving assistance.
Přijdu k tobě a ti pomůžu s úkolem.
I’ll come to you and help you with your task.
In this case, 'ti' is used as the dative form of the pronoun 'ty' (you), indicating the indirect object of giving the book.