An API call is a way to submit a request to an API endpoint on the client application side. It is to retrieve the requested data from the Credmark server and deliver back its output i.e. the response body to the client.
For this you will have to send a request after setting up your API with the correct endpoints and API key. Your information is then called, processed, and feedback is then returned.
Please find a detailed guide about the Credmark API here.