The Save command saves your image to disk. If you have already saved the image, the previous image file is overwritten with the current version. If you have not already saved the image, the Save command does the same thing as the Save As command: GIMP opens the File Save dialog, so that you can choose the data format, the path and the filename of the new image file.

If the image was previously saved, it is automatically saved to the same location, using the same file name, file type, and options. To save it differently in any respect, use either Save As or Save A Copy.

If you quit without having saved your image, GIMP asks you if you really want to do so, if the “Confirm closing of unsaved images” option is checked in the Environment page of the Preferences dialog.