Click Add Plan Description (or Edit Plan Description on an existing plan) in the Plan details card. A pop-up with a rich-text editor opens; the pop-up itself says "This description will show on the Subscription creation page". Type the text, click Save in the pop-up and then save the plan.

On the website the description is displayed under the plan buttons once the customer selects that plan. There is a separate Portion Description (next to the Portion Size field) that is shown when the customer picks the portion size.