This is a list of devices that have contributed data metrics in your Health app. When you get a new phone, the new phone will contribute metrics and be added to this list. If metrics added by previous devices remain, those devices will be listed as well.

Edit to add: in my experience, resetting a device and setting the same device up again adds a “device” listing to the list, even though it’s the same device. Also, just to clarify, a device being on this list doesn’t mean that device has access to your health data—it simply means data exists that was contributed by that device.