Saving and staging is the section after you have your project in a place where you're ready to watch it on your EEVO app. You'll find these options up at the top left of the composer under the "file" section.
Staging makes your current build available to the EEVO app without saving your current build on the composer. Saving overwrites the build you have previously saved on the composer, without updating what's viewable in the EEVO app. If you wish to save your composer session and overwrite the version saved on the composer, you can press save and stage.