Usually a phantom is a real physical object, with particular properties similar to a desired subject. The phantom is experimented with instead of the actual object.

Because you made the phantom, you have "ground truth". Whereas, for example, there is no way to validate your reconstruction of a living person's brain. Because you can't measure it without xrays, which might be the thing you're trying to improve.

But if you xray a phantom, you know what you should find.

I assume it's like phantom in the medical imaging context - a kind of test object.

The OED says: A physical or computer-generated model used to calculate radiation doses, evaluate or enhance imaging techniques, etc.