This is not so much an “Ask” as it is a “This might help”…
In the Building the Armature lesson for the Blob Man, I turned on the Axes option (right below the In Front option) of the Viewport Display section of the Armature. That showed the XYZ axes for each bone. Grant’s comments made more sense when I did that because I could see which way the axes were pointing when I changed the Roll param.