It allows you to quickly preview candidates in a selected recruitment stage. 

Go to Candidates section inside a particular recruitment, select candidates, and click Preview stage:

You will get a preview of their resumes with an option to add a note and tags and to rate each candidate. Here you can also access notes and ratings left by other users. To display the next/previous candidate, click Next/Previous at the bottom right of the screen:

When using the preview stage feature, you can also go to the Application subtab to see answers the applicants gave while filling out the application form: