M = Major hurricane. The eye (which is what is being forecast, not the geographic extent of the storm) could make landfall anywhere in the stippled area. There are indications Irene’s eye could be larger than usual, making the damage path (remember a major hurricane is being forecast) larger than usual, including electrical problems inland.
AccuWeather has more information here.
If you live in the stippled area, please take a look at some inexpensive preliminary safety ideas here.