It stands for "Advanced Placement." They're college-level high school courses. At the end of the year, you take the advanced placement exam, and depending on your scores and the college you attend, you can get college credits for them.
I think getting rid of an AP is a stupendously short-sighted idea. Having students take more advanced courses earlier is a great idea. If there's reason to believe the courses aren't actually as demanding as their college equivalent (and I don't think there is, based on my experience taking AP Calculus in high school and looking at what people taking Calculus in college were seeing. We covered the same material, and if anything my high school class covered more), then you can make an argument for the tests more challenging / add to the requirements of those courses. Getting rid of it is just an attempt to waste students' time and extract more money from them by forcing them to take more university courses.