Stop market order = when your price level is touched, your order becomes a market order. Stop at 1200 and price somehow drops a bunch quickly, you will exit at the current price. If you have a good connection and broker, it should not be too bad. Trading around news and all bets are off.
Most platforms default to stop market orders b/c a stop should mean just get me out!!!
A good example of your stop not being guaranteed in the least bit is to look at a daily stock chart with a big gap in it. People with stops in that 'gap area' will be triggered immediately on the open. No promises by anyone that your actual stop level be honored.
In the ES during normal conditions, you should be fine. You may get a tick or two slippage, but nothing drastic usually.