The purpose of Guideline 3.1 Make text content readable and understandable is to ensure text web content can be read and understood by the widest possible range of users with or without the use of assistive technologies.
The success criteria for this guideline require content authors and editors to take into account and address aspects of text presentation including language, unfamiliar or unusual words (jargon), abbreviations and acronyms, and instances where pronunciation affects the understanding of meaning.
There is one other success criterion associated with this guideline which addresses the reading level required for users to understand the text.
When text requires reading ability more advanced than the lower secondary education level after removal of proper names and titles, supplemental content, or a version that does not require reading ability more advanced than the lower secondary education level, is available. (Level AAA)
There are a couple of elements of this criterion that require explanation.
First, note that proper names and titles are excluded from this success criterion as it is agreed that it is not possible to modify such words without their meaning being lost. So, it does not apply to all text.
Second, this criterion specifies that an easy-to-read version of text is provided, as a supplement to the original text. So, the focus is more on providing alternate text that is easy to understand, rather than modifying the original text.
Lastly, this is one of the few WCAG 2.0 success criteria that refers to and depends on a specific external standard — in this case, a level of reading ability on the part of the user as defined in the International Standard Classification of Education (ISCE) adopted by the United Nations Educational, Scientific and Cultural Organisation (UNESCO).
That level is "lower secondary", which is defined as a reading level that can be expected to be attained after nine years of schooling.
The adoption of the ISCE standard is important. What it flags is that the WCAG Working Group could not identify a way of testing reading level successfully without referring to an external standard. Testability is a fundamental part of the WCAG approach: you must be able to test in order to meet the Success Criteria.
There are several techniques that are regarded as sufficient to meet Success Criterion 3.1.5 Reading level.