IDE Window: Desktops
Each desktop is a configuration of opened windows in IDE: window visible states and positions. For example, you can make desktop which shows Source Editor, Object Inspector and Messages windows.
"Debug" desktop is used when you activate "Debug" build mode in project options. For example, debug desktop can show additionally the "Call Stack" window and hide Source Editor window.
Dialog can be called from IDE's Tools menu. It allows to create/delete/rename desktops and choose any of them as debug desktop.