A readability score runs a formula, most commonly Flesch-Kincaid or Flesch Reading Ease, against a piece of writing and estimates the education level or reading difficulty it represents, based on measurable factors like average sentence length and syllables per word, giving writers a quick, repeatable way to check whether content is landing at an appropriately accessible reading level.
These scores are a useful proxy but not a perfect one: a text can score well numerically while still being confusing due to dense concepts or unclear structure, so readability scores work best as one input alongside genuine plain-language editing, not as a replacement for it.