Sentence fluency is one trait of good writing.
Sentence fluency builds on beginnings, lengths, and transitions of sentences.
It means that your sentences flow from one ideas to the next using transitions (See your Write Source 2000, p. 106 or look here or here).
It shows you can emphasize ideas with short and long sentences. Both simple and complex sentences add to the flow of writing.
It engages your reader by using different beginnings.
For more information, see the Writing Fix here.
Sentence Fluency to learn about sentence beginnings and transitions.
Review lesson: SF Review
Sample "Letters of Explanation"