While Power Automate and Power Apps offer over 320+ connectors to connect to Microsoft and non-Microsoft services, you may want to communicate with services that aren't available as prebuilt connectors.
Custom connectors address these scenarios by allowing you to create (and even share) a connector with its own triggers and actions and to expand extensibility of Business Central using Microsoft Power Platform.
A custom connector in Microsoft Power Platform is a wrapper around a REST API that allows Power Automate or Power Apps to communicate with that REST API. Connectors created in Power Automate are available in Power Apps. Likewise, connectors created in Power Apps are available in Power Automate.