Advanced Placement®, commonly called AP, is a program created by the College Board® for high and secondary school students to help them earn college or University level credits. This program enables students to take entry-level college courses during secondary school and earn college credit by scoring well on AP exams given at the end of the courses. Students are able to advance immediately to higher level courses as they start University saving money and also accelerating the pace at which they complete University. The AP program is recognized in the United States, Canada, and numerous other countries worldwide. The College Board defines AP as 

“A program that offers students the chance to tackle college-level work while they’re still in high school—whether they’re learning online or in the classroom. And through taking AP Exams, students can earn college credit and placement.” 

If you are a secondary  school or Sixthform Student s aspiring to attend college or University in the US , taking an AP Course gives you an edge with University admission process and also allows you to skip entry level classes and take higher level classes. If you want to earn college  credit before you finish secondary school you will need to do the AP course