Basic Syntax

Could any body help me understanding the basic syntax for creating plugin? . I am a beginner in Javascript and I know the basic syntax of javascript. But in " main.js" file, I am unable to frame the functions of every line of code.
Is there any suggestion on basic syntax learning for creating xd-plugin

If you want to learn about plugin structure, you should start with these document section. Also AdobeXD plugin samples repository has many useful examples on how to start with some basics (step-by-step).