Resource Library Contracts are one of the big ticket features in JSF 2.2. There's actually quite a bit to the feature - it allows for logical organization and packaging of the theme/look-and-feel/template for JSF pages. Beyond enabling better modularity, the feature makes it possible to switch page themes at runtime (much like PrimeFaces, RichFaces, etc did in the past). Huseyin Akdogan explains the details of resource library contracts in a code-intensive post. You should also checkout the video below from the GlassFish Videos YouTube channel in which specification lead Ed Burns covers the feature along with JSF 2.2 generally (the video is pretty short - just about fifteen minutes).
The official Java EE 7 Tutorial has a decent section on the basics of resource library contracts as well.