This is the 8425th most frequent German word.
katrin
"Katrin" is a German female given name; it does not have a specific meaning beyond being a name.
Here, 'Katrin' is used as a proper noun to specify a particular person's name.
Katrin ist meine beste Freundin.
Katrin is my best friend.
In this instance, 'Katrin' is the direct object of the verb 'getroffen' (met), used to indicate the person I encountered.
Ich habe Katrin gestern getroffen.
I met Katrin yesterday.
This sentence uses 'Katrin' in the possessive sense to denote ownership of the subject of the sentence, which is the book.