What is "Document Body" do in Pages?

With the Document Body present, you have a word processing document where you can freely type, and intermix document objects. Without a Document Body, you have an empty Pages Layout document where you must add document objects (e.g. Text box, Shapes, etc.) to construct a document. There is no free form writing as one would have in a word processing document.

If you have a word processing document with content, deselecting Document Body in the Document panel will warn you that you are about to destroy that content, and replace it with a blank Layout document.

Unchecking the Document Body in the Document panel gives you a blank sheet in a Layout document. You must insert Text boxes, and any other document objects that you position to form your finished document.