Use Apex REST API when you want to expose your Apex classes and methods so that external applications can access your code through REST architecture. Apex REST API supports both OAuth 2.0 and Session ID for authorization. See Apex Developer Guide: Exposing Apex Classes as REST Web Services. Connect your Salesforce to a third party application or connect a third party application to your Salesforce.
If you still need any kind of help, then please create a support ticket in your client area. We are always there to help you out. Moreover, if you need any kind of customization it is supported with your selected package.
