Connectors are used for integrating Micro-Apps with specific backend systems such as SAP, SharePoint or REST services. They have a design-time and run-time component. At design-time a connector can be used in the AppBuilder to browse backend system elements and types. Micro-Apps use Connectors at runtime for sending or receiving data from these systems.
Connector can also be used on Cards, to integrate card actions (e.g. approve/reject on approvals cards) with backend systems.