Can only be seen in presentation mode.
This icon is indicating that a media file is missing in your project. This icon is a helpful alert that ensures your presentation is complete with all the intended media. You should also notice this icon within your storyboard or timeline.
To resolve this issue, click on the icon. Doing so will allow you to relocate the missing file on your computer and restore it to your project. This process should help maintain the integrity and continuity of your slideshow.