Original referrers to the file you import into Aperture, the file made by your camera.
Managed and referenced refer to how Aperture handles the files, managed originals are stored in the Aperture library, referenced originals are stored outside the library.
To change managed originals into referenced originals select the images you wish to change and use the Aperture command File->Relocate Originals…
When you import a digicam file into the Aperture Library, you tell Aperture where to store it. If it is stored inside your Aperture Library package, it is referred to as "Managed". If it is stored outside your Aperture Library package, it is referred to as "Referenced". Aperture makes it easy to store your imported files on whichever directly-mounted storage device you want, and also easy to change the storage location at any time.
Once they are imported, your digicam files are called "Originals" in Aperture-speak. (In the manual, they also use the out-dated term "Master".)
For a quick review of the parts of Aperture, look here. I think you'll find it helpful.
Metadata is Image-specific. It remains with the Image, regardless of where you tell Aperture to store each Image's Original.
Lastly, to save space on your system drive, you might consider, as an alternative to referencing some (or all) of your Originals to an external drive, moving your Library to an external drive and leaving all your Originals managed. I have run Libraries off external drives for years.
Aperture program: on system drive
Your Library: either on system drive or on (fast) external drive
Your Originals: either in your Library (managed) or elsewhere (referenced), almost always on external drive.
I actually already had originals on my hard drive, but they were not 'referenced' in Aperture, they were duplicated there. This meant I had 2 versions of the original on my hard drive - a huge waste of space. (These were old photos that I had not pulled into Aperture from a camera).
The images in Aperture had metadata I did not want to lose.
When I 'relocated' the originals back into their duplicate file on the hard drive, the ones with no metadata overwrote the other originals, and those that had metadata created a 2nd image. I just deleted the one with no metadata.
It is a bit tedious, still, but luckily there aren't that many files I need to fix at this point.