The Close command closes the active image. If the image is not in the main window, then the window is closed also. If the image is in the main window, the image is closed but the window remains, empty.

Closing an image is not undoable: once it is closed, everything is gone, including the undo history. If the image is not «clean» — that is, if you have changed it since the last time you saved it — you are asked to confirm that you really want to close it. Note that an image is marked as clean when it is saved to a file, even if the file format chosen does not preserve all the information in the image, so it is a good idea to think for a moment about what you are doing before closing an image. If there is the slightest possibility that you will regret it, it can't hurt to save a copy as an XCF file.