Once you have an understanding of the Oomph folder structure, you'll see the need for using invisible rectangles in InDesign or more specifically the Shape Frame Tools. It's best to think of these invisible shapes as placeholders that will be replaced by text or an image later, depending on what feature you're planning on using. You can also use them to create invisible hotspots on top of content, for when you want an area of your content to link elsewhere.
To draw an invisible rectangle, from toolbox select the Rectangle Frame tool . Drag in the document window to create the frame. You can hold down Shift to constrain the width and height of the frame.
If you need to make your rectangle to be to specific measurements, select the Rectangle Frame tool and click once in the middle of your screen and you can then manually enter the dimensions for your rectangle.