This is a Header Document - a sticky on top of everything else.
Its title becomes the title of the Layout it is in - in this case, it is
the Gallery Title.
You can have such Headers in any kind of Layout, but they are not
necessary. If you delete it, you can always create a new one from the
Layout Picker.
To add images to this Gallery, just drag-and-drop
them in here (or click the Upload button). To edit the Header Sticky,
click it in the File Manager to navigate to it, then write something else
here.