The Zoom functions allow you to see the whole of images that are too large to fit on screen, and to see fine detail which helps when editing at a pixel level.
Zoom in zooms 100% in each time it is clicked up to a maximum of 1600%. Zoom out zooms out at progressively smaller intervals to a maximum of 6%. Note that for large images, zooming out may take a short while because GIMP has to load extra parts of the image from memory.
Default Keyboard Shortcuts: = for "Zoom In", - for "Zoom Out" and 1 for "1:1".