Much like PowerPoint, spell checker detects possible spelling and grammar errors, PowerPoint Accessibility Checker will detect possible accessibility issues within your presentation. Running the Accessibility Checker is a critical process in optimizing your documents and presentations for accessibility and should be done when you have completed your project.
Please visit Microsoft's documentation for Office/PowerPoint's Accessibility checker for more information:
https://support.office.com/en-ie/article/use-the-accessibility-checker-to-find-accessibility-issues-a16f6de0-2f39-4a2b-8bd8-5ad801426c7f