You can adjust the nesting level of slides to organize multilevel navigation in your presentation with the Promote and Demote buttons (green arrows to the right of slide thumbnails).
Available levels will increase each time you move a slide to a subsequent level. For example, when all slides of your presentation are assigned to the top level (1), you can add only one nested level (2). Once you move any slide to Level 2, you will be able to move the following slides to Level 2 or 3 and so on. Nesting depth of your presentation may reach five levels. You can collapse and expand nested slides using the "-" and "+" icons to the left of a top level slide.