What is the white piece of paper that models wear in their hair before a show and what does it do?

To see an example, take a look at the first few seconds of this youtube video: http://www.youtube.com/watch?v=tkup1Ng6U...

What is the white piece of paper that models wear in their hair before a show and what does it do?

It's so the hair artist knows which way the models hair should lay before a show. It also keeps the part in place making it more secure so it will hold throughout the show and all the back stage changes.

What is the white piece of paper that models wear in their hair before a show and what does it do?

That looks like foil paper for highlights.