The sidebar is the area on the right side of the Keynote window that shows the Format, Animate or Document inspector.
Format inspector: Click in the toolbar to show formatting controls for whatever is selected in the presentation, like text, a shape or an image. Click the tabs at the top of the sidebar to see more controls.
Animate inspector: Click in the toolbar to show controls for animating objects on a slide and setting transitions between slides.
Document inspector: Click in the toolbar to show controls for changing elements that affect your entire presentation, such as the theme and the soundtrack.
If a sidebar is open and you click its icon in the toolbar, the sidebar closes.