According to researchers, Marianne Meyer and Rebecca Felton (as reported on "A Focus on Fluency"), reading fluency is the "ability to read text rapidly, smoothly, effortlessly and automatically with little conscious attention to the mechanics of reading, such as decoding." For example, you can tell you are a fluent reader when you start reading a book and get absorbed in its story as you don't have to reread sentences to understand their meaning and you don't have the act of reading in your mind.
One important role of the text in developing reading fluency is that it allows readers to practice word recognition. When going through a text, a reader can see the word "fort" and remember it when seeing those four letters again. Through practice, word recognition becomes more accurate (the reader can recognize "meet" from "meat" for instance) and automatic, as readers can recognize words before they even finish reading them.
The more you focus on a cognitive task (recognition), the less you pay attention to another (comprehension), so texts can act as a bridge between the two stages. As reading practice makes word recognition accurate and automatic, readers turn their attention from the appearance of the word to its meaning. Only after understanding a word's meaning can readers make it part of their vocabulary and incorporate it in their speech.
Prosody refers to the intonation, rhythm and stress of the speech and it's one of the basic virtues characterizing a fluent reader. Texts help readers understand the role of punctuation marks, the pauses and intonation associated with them, but also allows them to practice reading aloud and test their rhythm. However, as texts can't give vocal examples of correct intonation, an instructor is necessary when you try to improve your prosody.