This is the 12071st most frequent German word.
udo
"Udo" is a German male given name, derived from Old High German meaning "prosperity" or "wealth."
In this sentence, 'Udo' is used as a proper noun, specifically a personal name.
Udo spielt gerne Gitarre.
Udo enjoys playing the guitar.
Here, 'Udo' is used as a name but with the definite article 'Die' to imply a specific individual known to the speaker.
Die Udo, die wir kennen, ist sehr freundlich.
The Udo we know is very friendly.
In this case, 'Udo' is part of a compound noun, referring to a person named Udo who is presumably the author or subject of the book.