The Master Preview displays a representation of the shader on the active Render Pipeline. It updates in real-time and automatically updates to display any changes you make in the Shader Graph. The title bar of the Master Preview displays the name of the current shader.
The Master Preview can be moved to anywhere in the Shader Graph Window and will automatically move with the nearest corner of that window.
You can rotate the preview mesh by holding left mouse button and dragging on the Master Preview and you can scale it by using the scroll wheel. The preview mesh can be changed by right clicking on the Master Preview. Here you can select from any primitive mesh types or select a custom mesh.