The Master Stack is the end point of a Shader Graph that defines the final surface appearance of a shader. Your Shader Graph should always contain only one Master Stack.
The Master Stack contains two Contexts: Vertex and Fragment. These represent the two stages of a shader. Nodes that you connect to Blocks in the Vertex Context become part of the final shader's vertex function. Nodes that you connect to Blocks in the Fragment Context become part of the final shader's fragment (or pixel) function. If you connect any nodes to both Contexts, they are executed twice, once in the vertex function and then again in the fragment function. You can't cut, copy, or paste Contexts.