Unlike the general SAT, which attempts to measure reasoning skills and college readiness, the SAT Subject Tests assess your knowledge in particular subject areas. They are content based, one hour long exams that test your knowledge of subjects on a high school level.
Taking the SAT Subject Test is a great opportunity for you to highlight your talent in a specific area of your choice and provide depth and variety to your profile. As with all the other parts of your college application, your Subject Tests give insight into who you are and what you like to study.