Data migration is the careful transfer of records, such as customers, products, open jobs and past invoices, from where they live now into a new system. It usually involves cleaning the data, matching it to the new structure and checking the result.
The risk is not the copying, it is the quality of what is copied. Duplicate customers, missing postcodes and inconsistent product names all follow the data into the new system unless someone deals with them first. A sound migration includes a test run, a reconciliation of totals, and a clear plan for what happens to the old system afterwards.