What is a Grails Plugin? What does it mean to Install a Plugin?

I used Grails recently, and added Grails plugin for JQuery, but I don't think it did anything more than just copy some jQuery files over.

So far, I have seen info only on 'how to install and use' plugins...but I can't find anything that describes the concept of a plugin.

Can somebody please tell me, what is a Grails Plugin? And what does it mean to 'Install' a plugin?