This is the 232nd most frequent Russian word.
эта
"Эта" means "this" (feminine form).
In this sentence, 'эта' serves as a demonstrative pronoun meaning 'this,' referring specifically to the noun 'книга' (book). It agrees in gender (feminine), case (nominative), and number (singular) with the noun it modifies.
Эта книга очень интересная.
This book is very interesting.
Here, 'эту' is the accusative form of 'эта,' used to point to a particular feminine noun 'женщину' (woman) as the direct object of the verb.
Где ты взял эту женщину?
Where did you find this woman?
In this example, 'эту' is also in the accusative case matching the feminine noun 'картину' (painting), acting as the object of the prepositional phrase 'на эту картину' ('at this painting').