Learning a framework is just like learning any language out there. You would have the same issue if you wanted to start to write an application with .NET, Rails, Java, Python, etc.
To me that opening paragraph is not jargon filled if you understand what Frameworks and JavaScript is about
MooTools is a
compact, [Small size]
modular, [Can be used in parts]
Object-Oriented JavaScript framework [OO JavaScript? :
http://java.sun.com/javascript/ajaxi...tion_ApB.html]
designed for the
intermediate to advanced JavaScript developer.
It allows you to write powerful, flexible, and
cross-browser [It deals with the browser quirks]
code with its elegant, well documented, and coherent API. [compared to other frameworks the api is nice]
You are not forced to use this thing [unless your boss says you are]. There are plenty of frameowrks out there from prototype to JQuery to Dojo and so on. There are 1000's of them. Find one you like.
Eric