This is the 8th most frequent German word.
die
"The" (definite article for feminine singular nouns in German).
In this sentence, 'die' is the definite article for 'Katze', which is feminine singular in German.
Die Katze sitzt auf dem Dach.
The cat is sitting on the roof.
Here, 'die' is the definite article for 'Pflanzen', which is feminine plural in German.
Ich habe die Pflanzen im Garten gegossen.
I watered the plants in the garden.
'Die' in this sentence serves as a relative pronoun referring back to 'Bücher', which is plural.