Name that thing!

I was cleaning out the trunk today and thought I would check the air in the spare tire, so glad I did too! I would suggest that anyone that has not had a chance to check your spare, please do. The last thing you want is to get a flat then go to put the spare and its flat too. Check to see it is properly inflated, sorry back on topic. It was at 20 psi so I inflated it and went to put it back when I noticed something attached to the jack. I had no clue what its for and reading the manual didn't help either (it wasn't mentioned or pictured).

The yellow thing attached to the jack, so what is it? And whats it used for?