Even a surprising number of college graduates were found to only read at less than college levels. To expect an individual to comprehend or otherwise understand a written communication that requires reading skills beyond that possessed by the involved individual can be a very major error.
Readability is a measure of the ease with which a given passage of text can be read and understood.
The only complete way to test readability is to give people a passage to red and then follow up with a test to see if they understood it. If a significant number do understand the passage, it may be generalized that many other people with about the same level of reading skills will understand it too. After finding a way to predict reading levels that agree with how people actually score on standardized passages, one should also be able to predict how well they will understand other reading material or similar difficulty. This is the rationale behind readability estimates. Almost all readability estimates use some measure of word difficulty and syntactic complexity as their main predictors.