The AFQT score is a percentile score. What does that mean? In 1997, a study, known as the "Profile of American Youth," was conducted by the Department of Defense in cooperation with the Department of Labor. DOD administered the ASVAB to around 12,000 individuals, ranging in age from 16 to 23. Your AFQT score is a comparison of how well you scored on the four subtests, compared to those who took the ASVAB as part of the 1997 survey. In other words, if you have an AFQT score of 70, that means you scored as well or better than 70 percent of those 12,000 folks.
One ASVAB sub-measure, the Armed Forces Qualification Test (AFQT)--which is just a separate analysis of the same tests that focuses only on four of the subtests--is also population-referenced, but its score from 1-99 is a percentile score. This means that if you score a 73, you scored as well as or higher than 73% of test-takers in the national sample.