A plan defines how much someone will pay, how often they'll pay it, and what they get in return.
The frequency and payment amount are determined by the plan's Periodicity. Every subscription is established by its associated plan.
By default, changes to plans do not change existing subscriptions. But flags do exist in the API call that will propagate those changes to subscriptions.
All plans are defined as members of a Plan Group. First you must create a plan group, and then you will be able to add plans within that container.