ASVAB is an exam taken by prospective military recruits that consist of several subtests in general areas such as mathematics and languages, and tests such as mechanics and automotive science. 

In general, anyone wishing to join the Air Force,  Army, Marine Corps, Navy, National Guard, or Coast Guard must use ASVAB before being accepted. You can look here at an online ASVAB course for best military aptitude test study guide.

ASVAB serves a dual purpose. First, your scores on specific language and math sub-tests form your (AFQT) score. The AFQT is used to determine whether you can enlist in the military. 

Recently, the minimum AFQT value required for enlistment has increased as more people are interested in joining the military. Second, ASVAB is used to qualify for certain Military Occupation Specialty (MOS). 

Subject tests can help you determine the types of military jobs you could have during your military career. Good grades on all tests will broaden your career options.

This is why preparing for ASVAB is so important. While some people mistakenly believe they can take ASVAB "cold", it usually doesn't work for their best results. ASVAB is an intensive test and requires prior preparation to get good grades.

Although many ASVAB books have several practice tests, they should not be canceled. Taking one exam right after another not only means losing the practical exam, it also doesn't teach your current position and increase your knowledge base.