Comment box is a special node. You can drag it around the workspace like a normal node, but you can also resize it. Every node that you place on it will follow if you move the comment box.
In the inspector, you can give the box a name, change its color, and use it as a bookmark.
You can create a comment box by pressing the C key. If you do this while nodes are selected, the box will be resized to contain all these nodes.