A basic question about "swap"

[kingpip]

Hello, I'm an absolute newbie to the world of forex. I have a basic question regarding a "swap" in MetaTrader 4. I've attached an image of what I mean.

http://img11.imageshack.us/img11/6203/mt4.gif (http://img11.imageshack.us/my.php?image=mt4.gif)


So what exactly is this swap? When does it occur? and how is it calculated?

[Customer Services]

Foreign exchange / precious metals trading (http://www.alpari.co.uk/en/new_to_markets/index.html) at Alpari (UK) is dealt on a "Spot" basis only. This means that all trades settle two business days from inception, as per market convention. Alpari (UK) does not arrange physical delivery of currencies / precious metals, hence all positions left open from 10:59:45 p.m. to 10:59:59 p.m. (London time) will be rolled over to a new value date. As a result, positions are subject to a swap charge or credit as specified in the table below.

Please note that since 03 June 2007 Alpari (UK) Limited no longer closes and reopens the positions which are open at 11:00 pm London time. Instead we have introduced a more convenient method of rollover which involves debiting or crediting a customer’s trading account when he/she holds open positions overnight.
For examples and detailed information, please review our Rollover and Interest (http://www.alpari.co.uk/en/new_to_markets/forex/rollover.html) web page in the New to Market section.