Otherwise, there are various readability statistics that attempt to quantify the readability. A high readability score means something is difficult to read.
Students being able to understand
Being able to understand students
The flipside of this is that students produce written texts in learner English, which will be read either by the teacher and/or other students and/or others. Students make errors which may reduce readability, such as spelling mistakes, grammatical errors, and poor handwriting.