This selection "zooms in" on the current bitmap or metafile. For bitmaps, each pixel is shown as a square. For metafiles, only a portion of the file is viewed, but at increased magnification.
It is useful for viewing details in an image. It does not modify the image itself – use Transform | Resize for changing the size of an image.
You can select Zoom In multiple times to view progressively smaller areas of an image. You may also use the keys 1-9 to zoom from 1x (unzoomed) to approximately 9x.
See also: View Window - View | Zoom | Out.