This button allows you to zoom in and out on the view. The three sizes are:
1:1 (original image size)
Zoom (zoom in on enlarged area
Area (zooms to full viewing area)
The Fill Mode Tool allows the image to be viewed in three different modes. These three modes are fit-to-fill, zoomed in, or preserve the initial aspect ratio.
Fit to Screen
This first view lets the image fills the entire screen
Zoom
This view allows parts of the image to be viewed closer
Preserve original aspect ratio
The third view preserves the original aspect ratio and keeps the proportions undistorted.