Key mapping basically stores the information that MATNR 4711 in one system maps to MATNR 0815 in another system, so these IDs are mapped during data import and replication. But I can't really provide a replacement for an MDG training here.
All our guides are in http://scn.sap.com/docs/DOC-7858#section2
There is a data import/replication section for material, and the documents on data import and data replication contain information on key mapping.