You can use the Speaker Notes format to add a speaker note to a slide. This is a textbox in the slide that teachers and authors can see, but is invisible to students.
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, speakers notes will show up with a black border around it, inside which you can edit the content. Only one speaker note will show up for each slide.