The Shader Editor is used to edit materials which are used for rendering. Materials used by Cycles and Eevee are defined using a node tree. Therefore, the main window of the Shader editor is a node editor.
A list of all shader nodes is available in the rendering section.
- Use Nodes
The Use Nodes setting is mostly a legacy setting and should always be checked for materials.
The Slot menu can be used to select the active material slot on the active object. The material selector to the right of it can change the material that is in the selected slot.
- Pin (pin icon)
The pin button will keep the current material selection fixed. When a material is pinned, it will remain visible in the Shader editor even when another object or material is selected elsewhere.