Canvas Groups is an editor feature that allows to group a bunch of nodes together for organization and commenting reasons.
To create a Canvas Group, simply click and drag in the canvas like when doing a multi-selection and then before release the mouse button, hold down Control.
You can then right click on the title to rename the group, delete it or select all of the nodes inside. You can have nested canvas groups within other canvas groups if you like.
If you want to move the canvas group without moving the nodes within for re-organization reasons, you can hold down shift while dragging the canvas group.