Polarizing Filters help needed.

OK soz for asking a somewhat basic question, but what is a polarizing filter for and more to the point would it be useful.

I think its used for increasing the amount of light entering the camera lens but thats a kinda guess.

The reason I ask is that I have been round a few camera shops looking for a telephoto lens for my S5500 and a couple of shops said I need a polarizing filter, and some said I don’t.

Thanks for any help.