Courses that are offered specifically to prepare students for freshmen-level college courses are known as what?

It's called Dual enrollment courses.

Explanation:

It enables the junior students to enroll for the next years' courses. High school students take up classes at local college and get to gain prior knowledge and college credit before entering the college.

This has many advantages. While still in high school, the student can gain an exposure to the routine and academic pattern in college so that the student frame his mindset to encounter the rigors of the college studies.

Taking prior class in the college, they know their area of interest well in advance. There are many virtual classes online which will enable the student to get ready for the college courses in advance.

AP classes, or Advanced Placement classes