What does it mean for a device to be supported to use ARCore?
It means a device has passed Google's certification process for using ARCore.
Here's what Google writes about it:
Certification is important because we want users to have a good experience with your AR application. This is primarily related to sensitive motion tracking, which is done by combining the camera image and the motion sensor input to determine how the user's device moves through the real world.
To certify each device, we check the quality of the camera, motion sensors, and the design architecture to ensure it performs as expected. Also, the device needs to have a powerful enough CPU that integrates with the hardware design to ensure good performance and effective real-time calculations.
The strength of Android is in the large diversity of devices available across the world. We are constantly working with manufacturers to make sure their hardware and designs meet these requirements. At the same time, we are working internally to make sure ARCore integrates well with every model we certify to provide good experiences for users.
And look at the full list of Google Supported Devices.
Hope this helps.