In order to go to almost any 4-year college or university, a student must take one of the following entrance tests:
Some of the more competitive schools also require that you take one or more SAT Subject Tests (aka SAT II) in addition to the SAT.  The SAT is a general test, where the SAT Subject Tests assess your abilities in specific subjects.  Some schools identify which subjects in which you need to test; others give you the choice.
If you took an AP class this year, it might be wise to go ahead & take the SAT Subject Test in that subject this spring before you forget some of the material. 
If you took Chemistry this year & did very well, you might consider the same advice.