The panel on the left side of the Slideshow module revolves around templates you can use (and define) to quickly set the style of your slideshow. The Preview section at the top of the panel isn't something I find particularly useful (Figure 1). It does play a version of the preview slideshow (discussed later), but it doesn't include the transitions, so it isn't a true preview of the final result. It also provides a preview of the overall layout for a given template when you move the mouse over the name of a template in the Template Browser section of the panel. I don't bother to collapse the Preview section because I don't tend to have a large number of templates listed, so there is plenty of room for it. But I don't reference it very often.

Figure 1: The Preview section of the left panel is primarily used for getting a preview of the overall appearance of the slideshow templates.
The Template Browser allows you to select a template for your slideshow (Figure 2). Templates are simply saved settings based on the options available to you on the panel on the right side of the Slideshow module. As I mentioned, you can move your mouse over the name of a template to get an idea of what the slideshow will look like in the Preview section. To apply a template, simply click on it. Doing so will cause all the settings in the right panel to reflect the saved settings for the template.

Figure 2: The Template Browser provides quick access to the saved slideshow settings you want to use for the current slideshow.