Reading Level
Reading level is a measure of how hard your text is to read, usually expressed as the school grade someone would need to follow it comfortably. Tools score it by looking at sentence length and word complexity, and most guidance pushes you to keep it low.
A lower reading level is not an insult to your audience. It widens your reach, helps people who are skimming or stressed, and supports anyone reading in a second language. It works alongside plain language as a measurable target you can check.