Accessing services provided by Esri
Some ArcGIS Online services run on a credit-based model. You purchase or otherwise acquire credits for your ArcGIS Online organization. When someone from the organization uses qualifying services, credits are consumed. The number of credits spent depends on the service. See Service Credits Overview for details on which services require credits and, of those that do, how many credits are consumed.
Credits have value, including monetary value, so it's important that your applications protect them and the usernames, passwords, tokens, and so on that are required to get and use credits. To help you handle this task, ArcGIS Online employs OAuth 2.0, an open authentication protocol. You can learn how OAuth 2.0 works within the context of Esri services by reading the Authentication help topics.
Accessing ArcGIS Online Services describes how to register an application, generate a token, and use the token token to access services.