Shader Graph is a visual scripting language that compiles to shader code. This is great for those who have never touched shader languages as well as those who have deep shader knowledge and want to quickly and easily prototype a shader idea they have.
You can open the Editor by creating a new Shader Graph asset and double clicking on it. Upon opening it, you will be met with 4 main sections:
Each tab can be dragged around and re-positioned however you'd like, so feel free to customize it to your liking.
You can create any nodes that are available to you by simply right clicking anywhere in the Graph or by clicking and dragging nodes into the Graph from the Palette.
Each node has a certain amount of inputs and outputs, each with their own types which are color-coordinated to determine if the value is a vector and how many components it has.
Some nodes may output a different type given the input types, depending on how the node chooses to handle such a thing.
Notice how the Multiply node returns a green Float value by default (given it's two Float inputs), but when a Color is input, the output becomes a yellow Float4/Color.
Once you have saved your Shader Graph, you'll see there is a compiled Shader file next to it, which can now be used as any other Shader when making a Material. You can also right click on it directly and select "Create Material" to instantly create a Material from it.