I'm not 100% sure, but I think that it means the effectiveness of a language, or the effectiveness of a person's' use of a language. For example, people say that learning Japanese is difficult, but when we consider that it's present and future tense are the same thing, this reduces a lot of waste and makes it more effective or "precise" to the learner. Whereas, English has so many different ways to produce sentences, and different structural formulations, this could be said to be wasteful, and not precise or effective. In a similar manner, when we talking about a person's "precision of language" I think we are again talking about the effectiveness of how they speak or write. By keeping things short and to the point but still generating sufficient meaning, they are being precise with their words. Ernest Hemingway could be an example of this precision of language. In both cases, I think it means more can be said in a language, and understood, with less words. I hope that helps! Interesting question!