Where slots are spoken utterances of the user, phrases are spoken by the voice assistant. You create a list of phrases in a similar way to that you create utterances for slots. See also How do I create an Intent?
To create a new phrase list, go to Create > Components > Lists, click Slot Types, and click the Add button ().
If you have a list of phrases, the voice assistant randomly selects from the list for more natural responses. For example, the following image shows a phrase list called Confirmation with three responses.
You can use the phrases like slots by placing the curly braces around the Phrase list name in the Intent Confirmation field.