You can use the Speaker notes format to add a speaker note to a slide. This note is a textbox on the slide that teachers and authors can see but that students cannot see.
To add speaker notes 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 "Speaker notes".
In the editor, the speaker notes will show up with a black border around them, inside which you can edit the content. While you can add as many speaker notes as you want, only the first one will show up.