A split-screen layout divides a page into two side-by-side areas. It is like a book opened flat, with one page for an image and the other for the words beside it.
A sign-in page might show a welcome message on one side and the form on the other. The two areas can balance each other while serving different purposes.
On a narrow screen, the panels usually need to stack. Make sure the most important information still appears first and that neither side becomes too cramped to use.