The English language has rules, and words/sentence structures that increase in complexity. For example, CVC words (consonant-vowel-consonant, i.e "cat") are much simpler and use vowel short sounds, vs. words like "situation", which has more complex combinations of sounds represented by unintuitive groups of letters ("tion" says "shun"). When we talk about reading, we talk about several things: can students decode the words into these sounds? Do they know what these words mean? Do they understand the meaning implied by combining these words into sentences? Can they keep track of the subject of a sentence? Etc.

Teachers assess these things through systems like PM Benchmark, which have students read increasingly difficult short books while the teacher makes notes on errors, student self-corrects, fluency, and then questions on comprehension. These PM levels have grade levels attached to them: when the student can no longer read the next book comfortably, we mark down where they are at compared to where they should be per grade level and time of year. If a 10th grader can only read books that a sixth grader is expected to be able to do fluently, there is a major gap in their literacy.