This command gives you a "WYSIWYG" ("What-You-See-Is-What-You-Get) display of what the current record will look like when it is printed. The Display takes the form and character formatting of the currently selected Layout. For example, if the current Layout is "Rolodex Card", the Display will show each record in rolodex-card format. The Display will also show whatever fonts, sizes, and styles go with the current Layout.
You can keep the Display window open at all times, and move it to whatever screen position you want. You can also re-size the Display window. If you want the Display to always open in the same place on your screen, choose "Window Persistence" (see "Preferences").
SUGGESTION: Keep the Display open whenever you are changing "Field Setup" settings. This way you can get immediate feedback to make sure that your record will look like you want it to when you print.