basically, your internet is (i assume) currently wired. you have a modem, and this is connected straight to your PC.
If you want wireless internet (or wi-fi), then you need some way of sending your internet connection over the airwaves, so that your laptop can receive it. it sounds like your laptop already has the receiving bit in it (many do these days). so to get wireless internet, you need something to send your internet connection over the airwaves.
something that does this is called a wireless router. a router is just a little box that splits your internet connection up so a group of computers can connection to one main connection. it sits between your modem and the rest of your computers. a wireless router is just the same thing, but it also transmits the signals over the air too, for your latop to pick up.
if you don't own a wireless router, then it seems like there is one nearby in your street (the wireless signals can reach a few houses, depending on brick thickness etc.). this person, as is said above, hasn't protected it, so you (and anyone else) can access their internet connection.