A Modal Dialogue window is a Popover window that usually carries a confirmation or alert. They position themselves foreground and don't move- which can be an annoyance if you need to see information behind it relevant to what decision you make in the modal dialogue.
For Gnome users, this is available in Gnome-Tweaks. Select the Windows option from the Left Pane and then the top option of the Right Pane will be "Attached Modal Dialogues" - set this to OFF.
Other Desktop users can use dconf-editor:
sudo apt install dconf-editor
Follow the path org -> gnome -> shell -> overrides -> attach-modal-dialogs and set to False.
Now Modal Dialogues will be draggable so you can move it out of the way when needed.