Much like the way calculating a crude death rate compensates for differences in population size, age adjustment compensates for differences in populations' age distributions.
Some counties have a much larger proportion of older people than others.
Since the probability of death is higher for older persons, a county with a large proportion of older people is likely to have a higher crude death rate than a county with a younger population. Age-adjusting the death rate is a way to make fairer comparisons between areas which may have very different age distributions. The result of age-adjustment is the rate that would have existed in that area if its population had been distributed the same way as in the standard population. Each age-adjusted death rate in this report represents the rate that would have existed if the population in that area had been distributed by age the way the population of the United States was in 1940. Therefore, it is a sort of artificial number to be used only for the purpose of making comparisons with other rates adjusted to the same standard.