What is meant by SAT, is it the exam system in US?

Yes it is an exam given to high school students to see if they are prepared to get into college. It helps the colleges to determine what level the student will enter college. It stands for Scholastic Aptitude Test. It is used along with ACT test which stands for Aptitude College Test. Maybe this will help clarify the two.

The ACT versus the SAT
The SAT dominates the national discussion of standardized testing; the ACT seldom gets mentioned. However, each year, nearly the same number of students take each test. Traditionally, American colleges used the SAT, rather than the ACT, as the standard-bearer in college admissions. But recently an increasing number of colleges around the country have begun to accept ACT scores from applicants, either in addition to SAT scores or in lieu of them.