"The feature is based primarily on statistical models we built with the help of teachers. We paid teachers to classify pages for different reading levels, and then took their classifications to build a statistical model. With this model, we can compare the words on any webpage with the words in the model to classify reading levels. We also use data from Google Scholar, since most of the articles in Scholar are advanced."
It seems unlikely that this will be of much value to English language learners, but it seems worth trying out. Here's this blog's breakdown by level.