I assume you know what an API is, but for those who don’t: it means Application Programming Interface, and it’s a collection of classes or functions in a programming language used to communicate (or “interface”) with other programs.
For you specifically: after 2 semesters of Java, I assume you know how to “include” libraries (I have no idea how this works in Java, so I’m using the terminology of C, which is probably different.) I’m assuming that there is such a library (probably a jarfile of classes) which provides the interface between Java and PL/SQL, similarly to how you would use Java to manipulate SQL. A few minutes’ googling wasn’t enough to find more than what I linked above, so I don’t think you should feel stupid because it’s not immediately obvious how to proceed. It took me a long time to figure out PHP/MySQL, which is a whole lot simpler.
The best advice I can offer is this: try to find a local mentor, ideally someone who has taken the class before and can help you through setting up a dev environment. Failing that, just keep hacking on it and believe that you’ll get it eventually.
All this stuff was invented by regular human beings to be used by regular human beings. It’s a massively complex problem domain and there’s a certain amount of non-reducible concept clutter to be sorted through and digested. It’s just a matter of time and skull-sweat.
Take heart! I’ve been doing this stuff more than 20 years, and you already know more programming languages than I do :^)