This is the 1st most frequent German word.
der
"The" (masculine definite article in nominative singular).
Here, 'der' is the definite article for the masculine noun 'Hund' in nominative case.
Der Hund bellt laut.
The dog barks loudly.
In this sentence, 'der' is the dative form of the definite article for the feminine noun 'Schule'.
Ich gehe zu der Schule.
I am going to the school.
Here, 'der' is the genitive form of the definite article for the feminine noun 'Tür', indicating possession.