-javadoc is a file that contains all the JavaDocs for the project, JavaDocs are a type of documentation that can be written into the code, and then generated into a web page, for example: jd.spongepowered.org
-source contains the source code for the project, it's basically a zipped version of the code you can see on the GitHub link. Java works by taking source code and translating it to byte code, and then byte code is actually run. The source code is provided so that.... well, so you can view the source...
Maven is a system that builds and deploys jars, and it usually includes the byte code (runnable) jar, javadoc jar, and source jar in its uploads. Also, literally everyone uses Maven, so yeah. (Or Gradle, which uses Maven protocol anyway) (Willing to bet me4502 is using Maven/Gradle or some other system that follows a similar structure).