The main things are event handling, DOM manipulation, selectors and AJAX. Everything else is just a build up of those things in addition to understanding fundamental principles such as; scope, binding, closures, etc. Without understanding some of the fundamental concepts all you'll ever be able to do with JQuery on your own is copy others code, apply a plug-ins and be a complete slave to the way jQuery does things, regardless of whether or not its the best way given a certain circumstance. Not to mention follow the lack-luster pattern of cramming 1000+ lines of code into a single function, with worthless comments if any.
There are already enough under qualified JQuery tools producing horrendous code, because they lack any fundamental understanding - don't be one of those.