A business service is part of InterSystems Ensemble interoperability production which is responsible for accepting requests from external applications.
I have been using IRIS / Ensemble for over a decade and appreciate lot of it's functionalities and features, however besides having a UI of 80's era (which doesn't bother me), what I believe where IRIS is lacking is lack of out of the box connectors (services/operations).
If we look at IRIS's competitors for eg Mulesoft, Talend, Boomi they all have hundreds of pre-built connectors for major applications like Salesforce, SAP etc and cloud services like Azure, AWS etc to store and retrieve information and data.