This is the 9430th most frequent Russian word.
ту
"Ту" means "that" or "there" (used in specific contexts).
"Ту" in this sentence modifies the word "задачу," specifying the task being discussed.
Ту задачу легко решить.
That problem is easy to solve.
In this sentence, "ту" is used as a demonstrative adjective meaning "that" and modifies the noun "девушку" to specify which girl is being referred to.
Я заметил ту девушку в парке.
I noticed that girl in the park.
Here, "ту" serves as a demonstrative adjective modifying "книгу," indicating which book should be taken.