API calls are a way to get data from external services or trigger some actions on the service's end (send a message, charge a card, etc.). Usually (but not always), the first type of calls are going to be GET requests, while the second one is a POST. A call usually requires some authentication, so that the service identifies who is calling it (and may charge per call, etc.). You add such calls in the API Tab of the Plugin Editor. This is where you will define how the calls are authenticated, and which calls are used as data sources or as actions.