This family of services is used to connect to external data sources and fetch data from them. This is just a base class which cannot be used by itself. Implementations are done for specific subtypes.
Connector services are designed to provide a standard way of connecting to remote data source. Every service is dedicated to querying a specific type of data source, but all services implement a common API.
Please refer to the manual for full details: https://docs.typo3.org/typo3cms/extensions/svconnector/