Synced is a property for slideshows, panoramas & splash texts. If it is set to true instead of creating their own slideshow / panorama / splash text they will use the one of the main menu. Therefore the panorama and the slideshow will continue to run when switching guis and the splash text will stay the same.
Notice that you set this property on the guis you want synced to the main menu, not on the main menu itself. Also if the main menu doesn't have a slideshow / panorama / splash text but you set it to sync in another menu your game will probably crash.
Notice that synced only works on a background slideshow, it will have no effect on an image that is a slideshow