The tutorial you’re reading is very old and things have changed. This doesn’t mean you can’t do it this way, it’s just a lot of unnecessary work.
However, the dotted lines show the parent/child relationships. The first bone in each finger and thumb group (the bone closest to the hand) is a child of the hand bone. Also, each finger has an IK Target bone at the end (the last big bone, not the tiny null bone) which is not really connected to the finger tip but is parented to the hand bone.
So, when the hand bone moves, the fingers, thumb and IK targets move with it and can then move independently.
The tiny null bones are the IK solvers - they are connected to the finger bones and as such, are children of those bones. These solvers pull the finger chains toward the Target bones. These were used because it was the base of the bone that was attracted to the IK target in the old Blender so to make the finger tip point where you wanted it to, you had to add an extra bone at the end. The big bone sticking out of the wrist is the target for the hand bone. No null bone was used here so the wrist follows the target and the hand can be freely rotated.
If you’re using Blender 2.4+…
IK has changed in 2.4 and you will not get the above to work without changes. You don’t really need null bones any more. You can use the finger tip bones and select “Use Tip” in the IK constraints panel. Or leave the null boness and disable “Use Tip”. Also, you will need to set the ChainLen for each IK chain to tell Blender how many bones to use in this IK chain (IK chains can jump gaps). For the finger chains set ChainLen to 4 if you use a null bone, or 3 if you delete the null and enable “Use Tip”.
I think that covers it. Good luck
Note: A child can’t have two parents but a parent can have many children.