Website migration means moving a website from one place to another — for example, from one Hosting provider to a better one, or to a new address — while keeping everything working and losing nothing along the way.
It is like moving house: you have to pack every item, carry it across, and put it all back exactly where it belongs, then update your address so letters still arrive. For a website that means transferring all the files and the Database, then pointing the DNS to the new home so visitors land in the right place.
Migration is done carefully and usually with a fresh Backup in hand, so that if anything goes wrong during the move, the original can be restored immediately.