Thick is a descriptor of a woman. It lets you know that she is not slim, and may be of above average weight or just a bigger build than normal. Thick does not mean fat or even chubby, it's the step between "average" and "chubby" and is not a bad thing.
"So how is that girl that wants to hook up with me?"
"Well, she's a brunette, nice face, a little thick, nice ass."