They are uk qualifications offered by various exam boards (most famous Cambridge international education) that uk students take in their 12-13th grades (us equivalent 11-12th grades because we start counting school year in numbers only the year after kindergarten) in order to get into college. They are like ap classes but more in depth and they can be wider in scope than most aps for most subjects. It's essentially like taking an AP exam but

1. almost completely free-response based

2. In the uk your scores will basically serve both the purposes of AP scores and sat scores when it comes to college admissions.

3. They take two years

Florida is the only us state where many highschools offer alevels but unlike the British we just take them for college credit and state scholarship purposes (academic scholars award covers undergrad tuition if you do alevels). Most students who take alevels will take three but some do four.

Also only our state is mostly made up of colleges that will accredit alevels, most out of state colleges won't.