The term ‘Migrate’ in the context of Drupal means re-creating the content and configuration of a source site (usually Drupal 6 or Drupal 7) in a new Drupal 8 installation. In earlier versions of Drupal, the common practice was to provide an upgrade path. i.e., update the code to the latest version and then upgrade the database structure according to the code. Once it is done, the content is updated separately.