question for a friend extra power cord with his macbook?

[hank smith]

hello my friend doesn't have internet access he wanted me to post this
his macbook came with 2 power cords
the one that goes in the wall and another cord. can any one tell me what 
this is?
he said it looks like you can plug another adapter in to it. he showed it to 
me and I have never seen anything like it.
thanks
Hank