The Braintree plugin charges credit cards and PayPal accounts. On mobile web, you can charge using Venmo, Apple Pay, Android, Pay, etc. You must create an account with Braintree before taking payments. Initially, start with an account in their Sandbox so that you can test, enter the keys in Development mode, and then when you're ready to go Live, you will copy the live keys. Non-sandbox keys are only applied in Live mode.