When editing a presentation, you can take Notes on the slides. Find this option in the page footer of your editor and just click the "Notes" button to be able to write a note for the slide.
Keep in mind that notes are not included in the exported version of the deck.
Access your notes when presenting
In order to read your notes while presenting your deck, use the Presenter View. Find it among the presentation options in the dashboard. Check this article for more instructions on using Presenter View.