samirs
this means that you are trading with a Market Maker (MM) which in most cases is the spread betting or cfd or FX provider with whom you have the account.
this means that if you buy the FTSE in £10 a point then Vice Versa the MM will have sold. (i.e have sold to you) if you make money the MM must pay you and if you lose then you pay to the MM. Many people do not like this as they believe that it sets up a conflict of interest in the company providing you with a service. for example if you win all the time it is hardly in their interest to offer you a service.
With a bookie at the races, if you just win too often, eventually the bookie will stop taking your bets or in extremis, if the bet is very large, 'nobble' the runner (or maybe even you!). Although I understand that quite a few of the big bookies quietly lay off the risk on Betfair or with another bookie.
Of course it does not really work that way in the financial markets as there is nearly always another client who has traded the other way (i.e sold £10 of the FTSE) and anyway the SB/CFD/FX company can always cover the risk by trading in the real market. (in the murky past though I am sure that there were some dogdy deeds going on)
these days though the financial markets are so heavily policed that such client negative activity is virtually impossible .. to be honest if one of the SB companies was doing it ... one of the others would probably shop them!
Simon