Glenn Fleishman, Macworld:

Apple took system protection to the next level in macOS 10.15 Catalina by splitting your normal boot volume into two pieces. It appears like a single volume on the Desktop, but it’s really two: one is labeled with the volume’s name, while the other has “- Data” appended to it.

Great explainer on the new boot volume setup, with a side mention of that “Relocated Items” folder alias you’ll likely see on your desktop when you install Catalina.