3G wireless cards

I see on the macbook pro product page it says

"No Wi-Fi? No problem. With the ExpressCard slot in MacBook Pro and a 3G wireless card, connect to the Internet anywhere there's a cellular network."

Do you need a monthly subscription to use these or do you just buy one, install it, and use it when you need it?

You need a subscription. They work off cell service. You can get them on discount from providers or just buy one somewhere and sign up.