If you click this icon then the screen splits in 2, with the list view still displayed on the left side of the screen.
The icon along the top of the screen also changes to reflect the fact it will now hide Split Screen mode.
If you click on any of the records listed on the left then that record will be displayed on the right hand side of the screen.
The icon along the top of the screen also changes to reflect the fact it will now hide Split Screen mode.