At typical X grader, given the same Y-grade-level test. So if it's a second grade test, and comes back as 5th grade, he did as well on second grade-level work as a 5th grader would be expected to, not "he's performing as well as a 5th grader would on 5th grade material," because they didn't test 5th grade material.
Yep, that is the explanation of grade level that I've known of. Our school doesn't use STAR math but they use STAR reading test to access kids' reading level. My son had reading tests every month last year. His reading level jumping between 4-8. I didn't take it seriously but I guessed the more reliable number is 6-7.
