Flatten

This command is a shortcut for flattening the image and thus merging all the objects within your project.

I strongly advise you to make a copy of your multilevel project before flattenning. Just to keep a copy of all the different objects that compose it. This command must be used only when you want to export your project as an object in another project. Even if you're sure that your work is perfect and finished you should still save a copy of your multi-level image ! Just in case ...

Learn that and more from the Objects palette.