"A mind without concentration is like a pile of wooden posts left lying on the ground for people and animals to step all over. But if we stand the posts up and plant them in the soil, we can get good use out of them. Even if they're not tall - only a meter or so - but we put them close together in a line, we can fence in our yard and prevent people and animals from coming in and traipsing all over our property. It's the same with the mind "

A nice article with MANY metaphors.