Okay, so a four-year junior college is a school you can go to after you finish high school. But instead of going to a big fancy university right away, you go to a smaller school for the first two years.
At this junior college, you take classes that are similar to the classes you would take at a big university, but they are a little bit easier and usually cost less money. After you finish those two years, you can either stay at the junior college and finish your degree, or you can transfer to a bigger university to finish your degree there.
The reason some people choose to go to a four-year junior college is because it can be a good way to save money and figure out what you want to study before committing to a bigger, more expensive university. Plus, some people find that the smaller classes and extra attention from teachers can be really helpful when they're just starting out in college.