This is the 453rd most frequent Polish word.
jan
"Jan" is the Polish equivalent of the English name "John."
'Jan' is a proper noun serving as the subject in this sentence.
Jan czyta książkę.
Jan is reading a book.
In this context, 'Jan' is in the genitive case ('Jana') due to the verb 'znać' (to know) requiring genitive for its object.
Znam Jana od wielu lat.
I have known Jan for many years.
Here, 'Jan' is used as the name of the person who is the subject of the future action described.