A material node is "hot" when the material in the node is assigned to one or more surfaces in the scene.
When you use the Slate Material Editor to adjust a hot material node, the material in the scene changes at the same time.
In the title bar of the node, the corners of the material preview indicate whether the material is a hot material:
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No triangles: The material is not used in the scene. -
Outlined white triangles: The material is hot. In other words, it is instanced in the scene. Changes you make to the material in the Slate Material Editor will change the material displayed in the scene. -
Solid white triangles: The material is not only hot, but is applied to the currently selected object.
A material is "cool" if it is not applied to any object in the scene.
To make a hot material node cool, clone it by using Shift+drag or Ctrl+Shift+drag. This makes a copy of the material (or of the material plus its maps) that is not used in the scene.