The Display view is not opened by default in the Debug perspective, so to open it go to Window->Show View->Other->Debug->Display. The UI of this view is rather simple, but it can be very useful, it gives you the possibility to:
- evaluate any expression and display its value
- execute given expression
- create a watch expression
The view can be used as a kind of scrapbook, where you can easily enter some elements of code and experiment a little bit while debugging.