Dire need in explanation for how a Certain part of Java Script works

I got a test in Java Script and the teacher does not teach us but instead gives us material and we need to self learn. I recieved last week a few Solution Files for a certain task in learning Java Script but i have sadly no idea how it works or how to replicate it. These files contain the code i need to learn. Can someone pls explain me how it works? I personally started to use Visual Studio but in "Class" i need to use Bluefish