Readability refers to how the style of writing used in a document or passage can affect how easily the reader can understand the meaning. The Flesch-Kinkaid Grade Level Score measures the relationship between the number of words in a sentence, the number of sentences in a paragraph and the number of words having more than two syllables. The formula determines the lowest grade level of student (or school year) that could understand a specific block of text.
Readability formulas measure quantifiable markers which can most obviously affect the difficulty of a passage of text. Readers often struggle with long sentences that can indicate overly complicated thoughts. They also may stumble over long words. People learning a language - either developing vocabulary as a child or when learning a second language later on - tend to learn shorter words first. Word familiarity is important as the more often someone encounters a word either in written or spoken English the easier it becomes to use that word and to understand the context. Therefore, writing short, active sentences with simple vocabulary will make using your site more rewarding for your users. Furthermore, readers will prefer simpler text when reading about unfamiliar subject matter.
The formula is not infallible. It cannot measure the actual coherence of the text. It can only provide a guideline as to its simplicity - assuming the author has written a genuine passage with correct grammar and punctuation. The formula can also indicate where potential problems might occur with the complexity of the text so that it can be re-examined. In all cases you should always follow an automated score such as the one generated by Flesch-Kinkaid with a human assessment. This can examine whether the text is properly suited for a specific target audience and how intrinsic any technical language is to the overall meaning.
Everything You Ever Wanted to Know About Readability Tests But Were Afraid to Ask.
William H. DuBay, The Principles of Readability,California: Impact Information, August, 2004.
Kim Fremont, Readability Issues in Training Materials for Adult Users, Edwardsville: July 27, 2003.