A data migration project with a lot of objects involving most of the modules - SD, FI, MM, etc can be termed as a large scale DM.
Value mapping is mapping the legacy/source values to the new target(SAP) values.
For example: A Company Code of "ABCD" in your current system would be migrated as Company Code "1000" in the new SAP system.
So when you are migrating any object that requires company code, the company code of "ABCD" would have to be migrated as "1000".