JSON is described best here: http://www.json.org/
It's like XML, but fat-free. You use it to move data around, store things, or just generally represent your program's state.
As a library, cJSON exists to take away as much legwork as it can, but not get in your way. As a point of pragmatism (i.e. ignoring the truth), I'm going to say that you can use it in one of two modes: Auto and Manual. Let's have a quick run-through.
I lifted some JSON from this page: http://www.json.org/fatfree.html That page inspired me to write cJSON, which is a parser that tries to share the same philosophy as JSON itself. Simple, dumb, out of the way.