In this tutorial on SitePoint.com author Daniel Sipos introduces you to the concepts behind the Migrate module in Drupal 8 and how it can be used to move content over from other sources.

Migrate is one of the most established modules in the Drupal ecosystem. So much so that with Drupal 8, a decision has been made to get some of its functionality ported and added to Drupal core. An important reason was that the traditional upgrade between major releases was replaced with a migration of Drupal 6 or 7 content and configuration to Drupal 8.

[...] In this article we are going to look at how migration works in Drupal 8 by migrating some content into node entities. For simplicity, the data we play with resides in tables in the same database as our Drupal installation.

He starts but outlining the "migration theory" and parts of the process to get the data in to your Drupal 8 instance: the source, the process and the destination. He uses a movie content example to show the setup needed to create the tables and define the configuration for the relationship to genres. He then shows how to make the migration configuration, defining the three parts and how to define the related migration classes. He ends the post with the drush command to execute the migration, get the current migration status and roll them back if something unexpected happens.