The slide navigator is a vertical list of slide thumbnails that appears the left side of the Keynote window in slide view. You can click thumbnails to jump to specific slides or drag them to reorganize your presentation.
In slide view you can do any of the following to show the slide navigator:
Pinch closed on the slide to reduce the zoom level to Fit or smaller.
Swipe right from the left edge of the screen.
Tap the button showing the slide number in the top-left corner of the slide.