When you delete a section in the builder, the icon will appear at the top of the layout sections. This allows you to undo and restore a section you have deleted by mistake.
You can ONLY undelete a section as long as you don't publish the App or switch to another App after you delete it. The undelete button only appears when you are able to undo the delete and disappears when you cannot.
If you delete multiple sections, the button will restore the sections one at a time with the latest deleted section restored first.