Live preview is a Chrome plug in that enables you to access the Live preview widget in the Translations studio. When you have finished translating your task you can see what your translation will look like live by clicking on the Live Preview widget in the Details side bar.
When you are on the live page you will see your language (highlighted in green) where the source used to be, and you can check if the layout looks right.
Click on the little arrow on the left to view each segment specifically. at the opposite end of the blue bar you will see two symbols. Click here to turn of the green highlight and here to toggle between your translation and the source text.