Conciseness is the extent to which a piece of writing communicates clear information in as few words as possible. One good way to think about conciseness is to think about what, in auto mechanics, is called the 'power-to-weight' ratio. 'Power-to-weight' is the horsepower of the car divided by the curb weight: in other words, the higher the ratio, the more powerful the car. But to achieve a good power-to-weight ratio, you must not only have a powerful engine--but you must also have to limit the weight of the car. Likewise, in writing, it's not enough to have intelligent things to say. You must find a way to 'lighten' what you're saying by using fewer, clearer words.