This is the 13512th most frequent Polish word.
pewną
The Polish word "pewną" means "certain" or "sure" when used in feminine singular instrumental form.
In this sentence, 'pewną' denotes an unspecific amount, used as a quantifier.
Mam pewną ilość pieniędzy oszczędzoną.
I have a certain amount of money saved.
The word 'pewną' is the instrumental case of 'pewna', used to describe being confident.
Jestem pewną siebie osobą.
I am a confident person.
Here, 'pewną' functions as an article-like descriptor indicating an unidentified woman.