Fluency refers to the ability to speak without hesitation. Whereas accuracy can be linked to grammatical understanding, fluency is more related to "language practice and skills work." (Cunningsworth, 1995: 19).
It doesn't necessarily have anything to do with a student's knowledge of grammar but more with how much at ease he or she feels using English. It's possible that students may find a tension between their struggles for greater fluency and their wish for improved accuracy.