When a juice has been concentrated, it's pasteurized and processed, then evaporated to get rid of most of the water content. It will later be rehydrated for consumption... They do this because juice lasts longer this way, but a lot of people consider it an inferior product to "not from concentrate" juice, which is considered fresher and to be in a more natural state, though the shelf life is lessened, and it often has to be refrigerated.
Re: juice question