In this case, the confidence interval refers to the range of values most likely to reflect the actual number of people who died as a result of the war.
To get at this number, the researchers made a best guess of 654,965, with a 95-percent confidence interval that ranges from 392,979 to 942,636. That means that if the study was done right, there's a 95-percent chance that the real number of war dead lies somewhere in that interval.
We call 654,965 the "best guess" because the actual number of dead Iraqis is most likely to be near the center of the confidence interval and less likely to be near the edges of the interval. As stated above, there's a small chance (5 percent) that it's not in the interval at all.