Lightroom’s Secondary Display option allows you to make the most efficient use of your screen real estate when you are working with your photos. Ordinarily, when you open Lightroom you are greeted with Lightroom’s primary window. This is where you do all your work on and with your photos and navigate between modules. This is the window every Lightroom user is familiar with.
Read Using the Secondary Display in Lightroom 2 over at peachpit.com.