The College Board's Advanced Placement (AP) courses are college-level classes in a wide variety of subjects that you can take while still in high school. They offer you challenging course work and a give you an example of what to expect in college.
According to the AP website, "AP helps hundreds of thousands of high school students achieve their college dreams each year." The courses will make you stand out in college admissions, help you earn college credit, and they will allow your to skip introductory classes in college.
For more information on AP, please visit the College Board's AP website.