Hemmingway (and DraftMap) use Flesch–Kincaid readability tests to metric sentences. Essentially, a breakdown of words and syllables into a formula produces an approximate reading level. Big words and bigger sentences flag as higher grades.

In case your interested, there’s offline program that does a similar task called DraftMap (demo at draftmap.com). The most popular feature is the repetitive word heatmap: tags overused words and helps you spot them from a distance. DraftMap marks a few other style issues, like cliches or the passive voice.