Fragments are used to show parts of the text on a slideshow, instead of everything at once. In other words, this content won't show up on the slide right away, but appears as the slideshow is continued.
To add fragments to your slideshow, first select the text you want to turn into a fragment. Click on the Format button in the toolbar. Select FORMATS, and select FRAGMENT. In the editor, the fragments will show up with a yellow border around it, inside which you can edit the content. You can add as many fragments as you want.