BreadwinnerNetSuiteAPI is a global class that can be used to make requests to QuickBooks via Breadwinner. As it is a part of the managed package, you must prepend the namespace breadwinner_ns.
The method .call(Map<String, Object> request) is a static global method within the BreadwinnerNetSuiteAPI class, which can be used to make an API request.
Breadwinner accepts the request data in the form of Map<String, Object>, and even returns the response data as a Map<String, Object>, thus making the requests and responses dynamic.
For more information, please refer to the Example Requests and Responses section.
Note: This is a synchronous method, it will make http callouts. So, to access BreadwinnerNetSuiteAPI.Call() method please enable callouts by annotating the future method, or use Queueable Apex. eg: @Future(callout=true)