Before getting into web animations, especially with javascript libraries like Greensock Animation Platform, you have to know the complete basics.
This includes HTML, CSS, and JavaScript. If you don't know these, you'll get lost, easily and quickly.
One of the main fundamentals you must learn is the DOM. In javascript, this is vital to you making DOM elements animate.
CSS is also needed because you have to know how to make things look a certain way to animate them.
I feel animation is key for user interface AND user experience. A good animation will resonate with the user, and make it easier to sift through your content.
The main goal in building website and web apps is the user experience. If you have terrible user experience, the user will leave your website or web app.