What does it mean when the yellow message "editing in X places" pops up?

The "Editing in X places" usually pops up for a moment when you are editing a node that is referenced in multiple places. It's a reminder that the changes you're making here reflects on X amount of other nodes in the graph.

To see all the other places this node is referenced, zoom into it and go to the Reference section at the bottom of the page. Here you'll find all the places in the graph where a reference of this node appears.