Shader Graph has nodes you can create, math functions, constant values, texture samplers. These nodes can be wired together and plugged into the inputs of the result node to change things like albedo, normals, roughness.
The preview will update showing you how the shader currently looks. When you save, a shader file will be generated ready for you to use in your materials.
New nodes are simple to create, this will be good if we ever decide to let users code their own graph nodes.
