This is the 4635th most frequent Dutch word.
duits
German.
In this sentence, 'duits' refers to the German language.
Ik heb een boek in het Duits gelezen.
I read a book in German.
Here, 'Duitse' is an adjective form of 'duits,' describing something related to Germany.
De Duitse bergen zijn mooi.
The German mountains are beautiful.
In this example, 'Duits' indicates the nationality of the person, but functions as a noun.