The Active Node is the node that is being edited. The Active Node is displayed in the Graph Viewport with a green fill color.
The Reader node is a selected node.
The Curves node is the Active Node.
The Split Channels node is the Display Node. The Green output socket is the Display Socket.
When a node is set as Active, the user interface is updated to include all of the node's user interface elements:
Select a node in the node graph.
Seleted nodes are displayed with white border lines.
Click the A key.
Using the keyboard shortcut has the advantage that the Active Node can be different to the Display Node, allowing you to see the effect of your changes in a larger branch of the Node Graph.