Select Float converts the current selection into a "Floating Selection". A floating selection appears in the Layers Dialog (Image Menu Dialogs Layers, Channels and Paths...) and can be manipulated in the same way as any other layer. To merge the floating selection back into the image, either click elsewhere on the image or right click the entry in the Layers dialog and select Anchor Layer. It can also be converted into a New Layer.
If you drag a selection, it will automatically be converted into a floating one.
Default Keyboard Shortcut: Shift-Ctrl-L