In SAP BTP, destinations are used to define connections for outbound communication from your application to remote systems that can be on-premises or in the cloud.
A destination has a name, a URL, authentication details, and some other configuration details.
In versions prior to v18 U 3 the EO GXEnterpriseSessionManager was used to connect to SAP, indicating all the connection data of the SAP ERP to which you wanted to connect.
Since v18 U3, you can provide a destination name instead and use it to connect via SAP BTP.
Se details in SAP server connection using GXEnterpriseSessionManager External Object.