A Draft App is any app that you create that is not activated under a paid plan. You can create an unlimited number of Draft Apps. Draft Apps have all the features of active apps but their install count is limited to 10 and you can't customize their Yapp ID.
During your free trial, any app you create is a Draft App. Once you subscribe to a plan, you may activate a draft app (or more than one, if you have purchased a plan with multiple app activations included). Once a Draft App is activated, it becomes an Active App, you may customize its Yapp ID, and the install limit is removed.
For more information, see how you can activate a Draft App or learn what happens when a Draft App has reached its installation limit.