Source: WebAim
PowerPoint contains two panels that can sometimes be used to enhance accessibility: the outline panel and the notes panel.
The outline panel contains a text outline of the content that appears in your slides. Reviewing this panel can help ensure the content on the slides is logically sequenced, that slide titles are unique and meaningful and that reading order is appropriate. The text in this panel may also be a good starting place for handouts or HTML alternatives to slides (more below). Alternative text for image and text boxes that are not part of the default layout will not be included in the outline view.
The notes panel allows the speaker to add notes and information that will not appear on the slides. It can be used to add additional information to printed handouts as well. Placing image or chart descriptions in this area is not reliable and should be avoided. This information may not be accessed by a screen reader, especially if the presentation is saved to PDF or some other format.