The recovery mode (also known as Safe Mode Boot) can be accessed when turning on the Nintendo Switch while holding + & - (volume buttons). This menu is made to update your Nintendo Switch through internet, if an update previously failed.
However it is possible to use this menu to delete the nagging update message if your console downloaded an update. It is believed that this menu simply erases all update data, as it should normally be accessed when the update files are corrupted.
This is called maintenance mode on 1.0, and recovery mode on 2.0