GenZedong

It's probably a measure of functional literacy, like the ability to read through a page of information and understand it fluently. Someone who is able to sound words out one by one wouldn't be considered functionally literate, for instance. They'd lack the ability to reasonably get through a text using reading as a tool. Instead reading would be an obstacle.

They might know a few words they need to know, like writing their own name, or reading some road signs, but they'd be unable to get through a novel or textbook. They might even know how to recite the alphabet or do math.

I used to teach adult literacy in the US and I've met hundreds of illiterate adults. Most of our studies showed 87% literacy but I could believe 79% too depending on the methodology and definition of literate.

Adults with the reading level of a small child, basically

English literacy skills sufficient to complete tasks that require comparing and contrasting information, paraphrasing, or making low-level inferences

Persons with very poor skills, for example, may be unable to determine the correct amount of medicine to give a child from information printed on the package

Have you seen the spelling and grammar on Facebook?

And it's often not a complete inability to read. They might recognise the shapes of words without knowing how to really read it, like when you go on holiday to a foreign country and start to pick up the local words for bus station or toilets because they're written everywhere.

People also tend to be able to hide illiteracy quite well, because they're embarrassed about it. The system failed them as a child, but has also convinced them it's their own fault.