Sun: The Java Extension Mechanism

Can someone please explain this to me in a sentence or two of english (even geek english) instead of Sunglish?

Update... Peter notes that the tutorial at the bottom of that page is pretty helpful. Indeed! I wish they had put that at the top of the page instead of the bottom. I got half way through the page and my eyes started to glaze over. (mind you it was just before lunch so maybe i was low on sugar ;-)) Thanks Peter!