Applications, built with Mendix, can communicate with SAP by sending and receiving IDocs, or by calling BAPIs on SAP.
IDoc = Intermediate Document. IDoc types are defined in SAP, and IDocs of these types can be sent to and from SAP.
BAPI = Business Application Programming Interface. SAP defines BAPIs which can be called from other systems (e.g. from Mendix).
Note 1: Calling a BAPI from Mendix is currently under construction.
Note 2: Mendix-SAP communication can also be done through webservices, which are described elsewhere in the Mendix documentation. Webservices are not available on all SAP systems.