It means that 95% of the values in the data set falls within 2 standard deviations of the mean value.
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if the confidence interval is 24.4 to 38.0 than the average is the exact middle: 31.2, and the margin of error is 6.8
What percentage of times will the mean (population proportion) not be found within the confidence interval?
Confidence intervals may be calculated for any statistics, but the most common statistics for which CI's are computed are mean, proportion and standard deviation. I have include a link, which contains a worked out example for the confidence interval of a mean.
You probably mean the confidence interval. When you construct a confidence interval it has a percentage coverage that is based on assumptions about the population distribution. If the population distribution is skewed there is reason to believe that (a) the statistics upon which the interval are based (namely the mean and standard deviation) might well be biased, and (b) the confidence interval will not accurately cover the population value as accurately or symmetrically as expected.
THe answer will depend on whether the confidence interval is central or one-sided. If central, then -1.28 < z < 1.28 -1.28 < (m - 18)/6 < 1.28 -7.68 < m - 18 < 7.68 10.3 < m < 25.7