what the purpose of maven site plugin

Does it generate javadoc site or to do with Ser开发者_StackOverflow社区vlet etc?

to produce reports and project info that can be published. all sorts of plugins can be hooked in to the site plugin through <reporting>(like the source code you mentioned)

See, for example:

http://maven.apache.org/plugins/maven-site-plugin/

All that content is generated with the ...

maven site plugin.