1. Content layer
This is where your slide content such as text and images live. Content is scaled to be as large as possible without affecting your original layout.
2. Background layer
This is the layer behind your content which contains the presentation background. The background is more flexible and scales to cover 100% of the width and height of the display to avoid letterboxing. Learn more about slide backgrounds.
Here's a visualization showing how these two layers play together to make the best use of available space. The tiled images represent the background layer and the light "SLIDE CONTENT" box represents the content.