Fragments are used to show parts of the content on a slideshow, instead of everything at once. In other words, this content will not show up on the slide right away, but appears as the slideshow continues.
To add fragments to your slideshow, first select the text you want to turn into a fragment. Click on the "Formats" button in the toolbar and then select "Fragment". In the editor, the fragments will show up with a yellow border around them, inside which you can edit the content.
You can add as many fragments as you want. The slide will first display all content that is not a fragment, with subsequent clicks revealing the fragments one by one.