What do icons mean on master slide layers?

Thanks Walt. I do see the thumbnails. I've determined that if there is no content on the slide, there is a horizontal line in place of a thumbnail. The slides in my master have triggers but no content. My confusion came when I saw a mix of square thumbnail boxes and horizontal lines (seen in my attached images). If I had no content on any of the layers, why did most layers have a thumbnail box like they did? The only difference between the layers with thumbnail box and layers with horizontal line is the transparency of the layer's background color.  Makes no sense but now I know these thumb/line icons do not matter for what I am doing.  
Thanks for your help!