This app is a readability tool which actually highlights the common errors, lengthy & complex sentences, grammatical errors and makes your writings more bold and clear. Basically this app removes lengthy complex sentences and modifies our writings into a more readable one.
So according to the website, when you see a sentence or line highlighted in yellow color that means you have to split it or edit. When it’s red in color your sentence is too dense and complicated for the readers to read it. So then you have to edit the sentence to remove the red color, otherwise your readers will lose their attention. Blue highlight indicates adverb and weakening phrases. Green highlights the passive voice.