Traditionally literacy statistics have divided adults into two groups, those who can read and those who cannot. This division is misleading as the level of literacy of adults is really a continuum. Many who would pass the 'literate' threshold may have difficulty with information involving complex tasks or instructions that are not clearly laid out.
In this study, moderate literacy is roughly equivalent to the skill level required for successful completion of secondary education.
Note that this statistic excludes people who function at a high level. See also Adults with at least moderate literacy