The Emission property is reflective of the light that an object receives. If the Emission is white it will be fully lit with that color. If black, it will act as if it doesn't exist. We make use of that by using black by default. However, if we do put our mouse over the object, the OnMouseOver function will trigger, causing it to use the color mentioned.
I could write much more on the subject of Shader Graph, but, unfortunately, there isn't enough space in the book. If you would like to explore Shader Graph even more, Andy Touch has put together a lovely group of examples of Shader Graph being used that could be great research material. Check it out at https://github.com/UnityTechnologies/ShaderGraph_ExampleLibrary.
