Market gapping happens normally because of major news that creates sharp price movements in a stock or other financial instrument as prices jump from one level to another, without trading in between. Markets can gap-up or gap-down due to several factors including factors including economic data, news on the economy, political news, or major world events.
Normal stop orders help to limit your downside risk but these are not guaranteed. While stop losses can be used to reduce the risk by closing losing trades they are not infallible and in situations where the market jumps past your stop, the spread betting provider will close your trade at the best available price once the stop value has been breached. This means that due to market gapping, occasionally your spread trade could be closed at a price level that is different from your trigger value. Thus, if the market does gap your closing price might differ from the trigger value you have set and thereby sustaining a bigger loss than your original stop loss dictated.
You can use guaranteed stops to protect your spread trades against the dangers of market gapping. These work in very much the same way as Standard Stop Loss Orders, but they guarantee to close your spread trade at the trigger value you have set, irrespective of underlying market volatility and gapping.