A splash screen is a page on which an image is displayed in full format. No navigation elements are available to the user - as soon as the image is tapped, a (predefined) next page appears. The splash screen page can therefore be used as a start page, for example.
In the page settings(xpedeo section), you can specify which page you want to continue with after selection.
The image is simply inserted into the page layout with an image entry.