The first set of items on this menu bring up dialog boxes that apply various filters or effects to the image. If you have selected a portion of the image with the selection tool, or by other means (for example, when you draw something in selection mode), then the effect will only apply to the selected image data.
Each of these dialog boxes provides an "Apply" button and a "Revert" button. Once you've chosen the options you want, click "Apply" to see how it will look. Don't worry about choosing the wrong optionswhen you apply the settings, the original image is stored in an offscreen buffer which can be restored by clicking "Revert". Clicking "Done" will exit the dialog without modifying the image further.
If you made a change to the entire image, close the dialog box, and decide that it wasn't what you wanted after all, you can still undo the operation (if Enable Undo was checked) by choosing Undo/Redo from the Edit menu. If you made the modification to a selection, the operation can't be undone; the best you can do is delete the selection (Edit | Clear or Ctrl+Delete).
Alpha blending options are only available when something is selected. These options are not enabled just by selecting a portion of the image; you have to have something on top of the imageselected text, for examplethat is not a part of the image.
The purpose of Flip Horizontally and Flip Vertically is obvious enough... the image, or the selected image data, will be mirrored accross the x or y axis.