what does iPhoto or Aperture library that you already migrated mean?

If you had a library created with either of those old applications (iPhoto or Aperture), it is in an old format. To use those photos in the new Photos application, a new library has to be created in the Photos format with the same photos. This "migration" happens when you try to open the old library with Photos. After that, you would use Photos on the new Photos library. The old iPhoto or Aperture library is then said to be "migrated" and is no longer used. Once you are happy with the results of the migration, the older library can be removed.

Does that help any?

If you open an Aperture Library (for example a library named "Aperture Library.aplibrary") in Photos for Mac, Photos will create a new Photos Library from it, in the same folder as the original library named "Aperture Library.photoslibrary". The original Aperture Library will be renamed to "Aperture Library.migratedaplibrary". The changed filename extension is the only difference. Your Aperture Library is still the same Aperture Library, but Aperture and Photos can tell by the extension, that you already have created a Photos Library from it. You are now having two libraries.

The same goes for iPhoto Libraries. A migrated iPhoto Library will be renamed from "iPhoto Library.photolibrary" to "iPhoto Library.migratedphotolibrary" and the new Photos Library will be called "iPhoto Library.photoslibrary".