This is the 154th most frequent Lithuanian word.
tas
"That" (demonstrative pronoun).
Here, 'tas' (in its form 'tą') acts as a demonstrative pronoun in the accusative case, specifying a particular book.
Padėk man atnešti tą knygą.
Help me bring that book.
In this sentence, 'tas' functions as a demonstrative pronoun referring to a person being described by the relative clause.
Tas, kuris tai padarė, buvo labai drąsus.
That one, who did this, was very brave.
In this sentence, 'tas' is used as a demonstrative pronoun to specify the particular man being referred to.