If you support drag and drop in your application, and want to start a drag and drop operation after the user has moved the cursor a certain distance with a button held down, you should use this property's value as the minimum distance required.
For example, if the mouse position of the click is stored in
if ((startPos - currentPos).manhattanLength() >= QApplication::startDragDistance()) startTheDrag();
Qt uses this value internally, e.g. in QFileDialog.
The default value is 4 pixels.