The Armed Services Qualifying Test (AFQT) is designed to determine your eligibility to enlist in the Armed Services. The score of the AFQT is determined by your performance on the Word Knowledge, Paragraph Comprehension, Arithmetic Reasoning, and Mathematics Knowledge subtests of the ASVAB. The AFQT is one of the most important parts of the ASVAB. It isn't a separate test — it's a combination of the scores from whatever version of the ASVAB you take.
Why is the AFQT so important? Your AFQT score is used to determine whether you can be admitted into the Armed Services (and which branch you're eligible to enter). Your AFQT score also affects which jobs you're eligible for in the Armed Services. So it's not enough just to get the minimum score — you want to get the highest AFQT score possible, so that you have a variety of job options once you're in the military.