"말을 잘 안 하다" can have different meanings depending on the context.
It could be used to describe someone who's not talkative.
e.g. 그 학생은 말을 잘 안 해요. The student doesn't talk much. The student is quiet. The student is not talkative.
Or this could be also used when there isn't much conversation going on between two or more people.
e.g. 우리는 서로 사실 말을 잘 안 해요. We don't actually talk to each other that much.