If you are an existing Billing user and do not upgrade to the Prices API, there is no action needed from you and your integration will continue to work as expected.
You can now model your business and product offerings in one place. Products define what you sell and Prices track how much and how often to charge. They can be used for recurring or one-time purchases and support various business structures from tiers to usage based billing. This is a core entity within Stripe that works with subscriptions, invoices, and Checkout. See more here.
You have 2 options available:
Do nothing. You can continue to use Plans, as the Stripe API is backwards-compatible.
For new offerings, use Prices. For existing offerings, pass in Plans into the new API. Read our Prices Migration Guide for more information.