Moving existing information
A company replacing its customer system may move names, addresses, and orders to a new one. It might also change how the same information is stored in its database. In either case, the records must remain complete and mean the same thing.
A backfill fills in missing values in older records, such as adding an account type to existing customers.
Check before and after
Teams test the move on a safe copy of the data, then check that records, dates, and amounts came through correctly. They plan how to keep the service running and how to recover if something goes wrong. Backups help, but undoing a migration gets harder once people enter new data in the changed system.