You can get an assessment of your essay by clicking on "Planning and Review ", "Collocation-Readability Check ":

After a few moments (depending on the length of your text), you will see that your text is colour coded as in the example below.

This colour coding gives an indication of the readability of your text, based on word collocation strength. At the bottom of the page there is also a key to this colour code and two readability scores:

The CR and CR2 indices give measures of the readability of your text based on collocation and word frequency scores.

For more information on these scores, see the help files.