A large number of college courses are available for high school students to take both online and in-person. Some students take them just to challenge themselves further or learn about one topic that isn’t offered at their high school.
College courses alone won’t get you into college. Taking 20 college courses won’t reverse your mediocre GPA or average ACT scores. That’s just not how it works. It can, however, add depth to a specific part of your application.