Cable disconnected. what is it for?

Hi,

i had to change my battery. which went well.

My problem is that when closing the case, a cable got disconnected and I dont know what it is for.

The broken end is here: http://i.imgur.com/JoP6iPX.jpg

As far as I can tell the phone works fine, except for bad reception. I just want to know what the cable is for.

Thanks for your help.