Each node has up to three icons on its top right hand corner. The icons are described below, starting from the farthest on the left:
Groups
Click the Group icon 
on the top of a group node to see its group contents.
To exit out of group mode, click Exit Group in the upper-left corner of the node graph.
right-click and choose Toggle Advanced Mode). This icon is always available for groups you create. It is available only for nodes with an internal network; for example, group nodes, or a texture node. Preview swatch
Click the Render Swatch icon 
on the top of each node to cycle through displaying: its icon, its small render swatch, and its large render swatch.
In the following example, clicking the Render Swatch icon a first time shows a small preview of a TextureMap node. Clicking Render Swatch a second time shows a larger preview swatch. Clicking Render Swatch a third time returns to displaying the node icon.
This functionality allows convenient visualization and debugging of your shading graph.
Left: Click once to show a small preview swatch.
.
Middle: Click twice to show a larger preview swatch.
Right: Click a third time to return to the default: displaying the node icon.
View mode
Click the Hide Inputs And Outputs icon 
on the top of each node to toggle between displaying the node attributes and hiding the inputs/outputs for a more compact node view.
R
Nodes that are referenced from disk are marked with an R symbol.