It's something a black walnut tree produces that supposedly kills most plants around it. If I were to list all of the plants growing under my black walnut, which shades almost the entire back quarter of my garden, the list would be as long as your arm, so I tend to think it's just another convenient excuse for plant death. The only thing I don't grow under there are succulents and hibiscus, but that's because of the shade, not the dread juglone.
Here's a photo of the trunk of my black walnut. That's a 5-gallon container standing next to it for scale, so you can see it's big enough to do a lot of damage if the stories were true.