Hello and again this doubt has to be explained to my ten year old. It's about JWS (Java Web Start).

Could you please explain the red-circled stuff in an easy way ( for my ten year old )?
He has no ( no! (or) zero (or) 0 ) knowledge about XML and blah blah blah.

He also does not ( nor do i ) know mime types but he know HTML.

I can't read the text in the attachments, but anyway...
XML is the generalised markup language - HTML is one form/subset of XML. There's all the info you could possibly want on the web, at every level from complete noob to ultimate expert, so I'm not going to try to add to that.
Mime types are a system-independent way to identify file types (eg text, graphics/jpeg) etc. They map to file extensions in Windows. They are used in web browsers/HTML in preference to windows file extensions because they are system independent. Once again, it's all on the web.

in the zip, please leave the "jnlp and error.bak" file alone. do not modify it.