UI for core upgrades from D6 or D7 to D8 - the plan is for this to live in contrib for now and ultimately be brought into core (a decision will be made shortly before RC whether it will make it into 8.0.0).
The UI is being designed to replace upgrade.php (which no longer exists).
After installing, visit /upgrade in your browser to access the interface.
Since the purpose of this module is to replicate the upgrade.php process it is designed to be as simple as possible. For this reason, it will attempt to run all relevant migrations known to the system.
To see a list of migrations known to your Drupal 8 system, you can use drush-7.x to see the list of migrations by typing
drush config-list|grep migrate at the command line.
Module developers may create migrations for contrib modules by creating custom migrate plugins. Talk to someone in #drupal-migrate for details or visit http://groups.drupal.org/imp.
As of May 11, 2015 only D6 migrations are in core. Work on D7 migrations is currently underway.
As of May 11, 2015 this module is working with current core (and has been for most of the past year) - it may break temporarily, of course, when core migration support changes out from under it.