The preview displays the model almost exactly how it will look like in SketchUp, with textures and opacity. It doesn't display the complex material features like reflection, refraction, etc, as this is handled slightly different by each rendering engine.
Show edges displays the edges of the model.
Know issue: for now the model is always triangulated in the preview. But the exported model will not be triangulated, so you may see more edges in the preview than what will be actually created in the SketchUp file.
The Zoom Extents button centers the model in the view and adjusts the zoom model so that it fits the window.