It's not one sentence, it has three parts :
-The first part ("كريم، من خير النَّاس وأفضلهم ") is a defenation of word "حر", that means free person who is generous, one of the best people.
-The second part (نساء حرائر) is an example of free people, and it means free women. Also we can say: رجال أحرار = free men.
-And the last one (أَوَ تَزْنِي الحُرَّةُ ) is an example was given for free women, it means: does the free woman commit adultery?. It was a question, and it was asked to the prophet Mohammad (pbuh) by a woman, when he was reading the aya of "يَا أَيُّهَا النَّبِيُّ إِذَا جَاءَكَ الْمُؤْمِنَاتُ يُبَايِعْنَكَ عَلَىٰ أَنْ لَا يُشْرِكْنَ بِاللَّهِ شَيْئًا وَلَا يَسْرِقْنَ وَلَا يَزْنِينَ" she interrupted him Wonderingly and asked him: أَوَ تَزْنِي الحُرَّةُ؟.
([حديث]: العفيفة المحصنة) is a reference for the speech.
Hope this helps.